Celebrim
Legend
One important aspect of low level encounters at high level is that they reinforce that the players relationship to the world has changed. What was once a terrifying encounter gradually becomes something that is not even relevant.
And also realistically, the world needs to recognize that the PCs have changed. Goblins bandits that previously charged into to rob, enslave, and eat the party are going to come to realize that this group of creatures is out of their league and their relationship is going to change. Instead of attempting to ambush the party they are going to attempt to evade or grovel and lick boots. As soon as the goblins realize they are in over their head, they are going to either flee or bow down and ask forgiveness with a lot of florid flattery: "Please do not slay us oh mighty slayers. We didn't realize it was you, great lords. Only allow us to live and we will serve you, bear your most unworthy burdens, clean you most noble clothing. We will be as your slaves, oh most mighty ones."
Also keep in mind that it's often a great time to change the way the PC's look at the world by having the PC's start encountering things when those things are about affairs that have nothing to do with the PCs, and force the PCs to decide how they want to relate to the world. Are they universally hostile to things that aren't hostile to them? Or are they going to consider trade in goods and information with beings that they formerly looked at only as foes?
You as a DM are not locked into trying to make the game work the same way at 10th or 15th level that it did at 1st level. The math does not need to work the same. The world does not need to work the same. The sort of encounters that the PC's have do not need to play the same.
That said, there are basically two sorts of low level enemies - those that become irrelevant and those that can still drain relevant resources. The first sort you will struggle to do anything with but Role Playing encounters at high level, but the second have what I call 'level invariant attacks'. Level invariant attacks vary from edition to edition and I'm not an expert in 5e, but in earlier editions they include things like: attacks that bypass AC, attacks that still do half-damage on a saving throw, attacks that don't get a saving throw, attacks that bypass hit points, and so forth.
A skilled DM should keep a list of the sort of things in the edition that are level invariant. For example, in many editions a splash weapons like burning oil bypass armor to a large extent so a low level creature with a vial of flaming oil can still drain resources. In many editions siege weapons bypass armor to large extent, so a crew of low level creatures with a siege weapon in the right situation can be a problem, as can low level creatures that can make any sort of trap. High level PC's are often relying on area of effect attacks to quickly clean up encounters with low level creatures, so if you instead spread the low level creatures out widely in all directions around the party, then the PC's attacks tend to be overkill. Grappling attacks tend to bypass the normal attack process to a large extent in many editions, so one possibility with low level creatures is that they just try to swarm over the PC and overwhelm them with numbers in a grapple. Each grappler may have a low chance of success, but if you allow 8-10 grapplers per PC eventually they may get lucky, and at the very least it's usually hard to cast spells and maintain concentration in a grapple.
At higher levels you have to up your game as a GM and design more involved encounters. Not only do you need to do this to keep the pressure on the PCs, you need to do this to keep experienced players from being bored. There is only so many times that you can be ambushed by monsters who have no apparent motivation on an empty stage with no terrain, who are attacking you by trying to close the range, get into melee and hope to survive to get in a few hits, before as a player you are completely bored. You might get through one campaign with a diet of that, but by campaign two you are going to need to improve your encounter design and add in weather, terrain, tactical variations, encounters with combined arms teams, three sided encounters, complex multi-stage encounters, encounters where either the PCs or the NPCs have particular goals in addition to eliminating the other side, and so forth.
Beyond changing tactics, you can also change what you use as a mook. In earlier editions I was always a big fan of mephits because they were pretty weak but they all tended to have level invariant attacks, ranged attacks, and flight - a combination that is perfect for a mook at higher levels.
As for the first solution that has occurred to you, it's not bad but I would suggest instead that you create a feat that encapsulates the idea of mooks swarming around less numerous foes, and then assume their are 'elite goblin light infantry' that have additional HD and this feat. So for example, I have a feat I often give to certain foes that doubles any flanking bonus - wolves for example have this feat as a racial bonus feat, and goblin infantry often has the feat. There is also a homebrew feat in my game called 'Pack Tactics' that gives you a bonus to hit and AC for each adjacent ally that is also adjacent to your target. For 5e, with its slightly different feat design, I'd probably combine the two ideas into a single 'Pack Tactics' feat. So at high levels you'd be encountering not just jungle goblins, but elite jungle goblin infantry with higher Dex, an extra HD, and the 'Pack Tactics' feat that gave them double the normal bonus for flanking plus a +1 bonus to AC and To Hit for each ally adjacent to them which was also adjacent to the enemy and which also had the 'Pack Tactics' feat. Then I'd design the encounter so that their was a covered spiked pit trap between the PC's and the obvious line of approach to the main body of forest goblins, and where 1 in 4 of the forest goblins also had a vial of flaming oil, and where several forest goblins had vantage points up in trees (with partial cover and partial concealment) from where they could snipe at the party with poisoned arrows, and where the forest goblins were supported by a low level spell caster such as a cleric or a bard. Or I'd have the jungle goblins try to throw weighted nets on the PC's, and have them set up to brachiate through the trees to engage in running battles where they took the high ground and had platforms with rocks on them that they'd throw down at PCs or containers of hornets that would break into mini-swarms, or vials of vegetable oil they'd break against tree boles to grease any player attempting to climb up to them.
Or have encounters like that in the dark during a thunderstorm with a flash flood going on.
Finally, don't let the dice ruin your game. If you ever roll a random encounter that you can't figure out how to make interesting, ignore it or select a different one.
And also realistically, the world needs to recognize that the PCs have changed. Goblins bandits that previously charged into to rob, enslave, and eat the party are going to come to realize that this group of creatures is out of their league and their relationship is going to change. Instead of attempting to ambush the party they are going to attempt to evade or grovel and lick boots. As soon as the goblins realize they are in over their head, they are going to either flee or bow down and ask forgiveness with a lot of florid flattery: "Please do not slay us oh mighty slayers. We didn't realize it was you, great lords. Only allow us to live and we will serve you, bear your most unworthy burdens, clean you most noble clothing. We will be as your slaves, oh most mighty ones."
Also keep in mind that it's often a great time to change the way the PC's look at the world by having the PC's start encountering things when those things are about affairs that have nothing to do with the PCs, and force the PCs to decide how they want to relate to the world. Are they universally hostile to things that aren't hostile to them? Or are they going to consider trade in goods and information with beings that they formerly looked at only as foes?
You as a DM are not locked into trying to make the game work the same way at 10th or 15th level that it did at 1st level. The math does not need to work the same. The world does not need to work the same. The sort of encounters that the PC's have do not need to play the same.
That said, there are basically two sorts of low level enemies - those that become irrelevant and those that can still drain relevant resources. The first sort you will struggle to do anything with but Role Playing encounters at high level, but the second have what I call 'level invariant attacks'. Level invariant attacks vary from edition to edition and I'm not an expert in 5e, but in earlier editions they include things like: attacks that bypass AC, attacks that still do half-damage on a saving throw, attacks that don't get a saving throw, attacks that bypass hit points, and so forth.
A skilled DM should keep a list of the sort of things in the edition that are level invariant. For example, in many editions a splash weapons like burning oil bypass armor to a large extent so a low level creature with a vial of flaming oil can still drain resources. In many editions siege weapons bypass armor to large extent, so a crew of low level creatures with a siege weapon in the right situation can be a problem, as can low level creatures that can make any sort of trap. High level PC's are often relying on area of effect attacks to quickly clean up encounters with low level creatures, so if you instead spread the low level creatures out widely in all directions around the party, then the PC's attacks tend to be overkill. Grappling attacks tend to bypass the normal attack process to a large extent in many editions, so one possibility with low level creatures is that they just try to swarm over the PC and overwhelm them with numbers in a grapple. Each grappler may have a low chance of success, but if you allow 8-10 grapplers per PC eventually they may get lucky, and at the very least it's usually hard to cast spells and maintain concentration in a grapple.
At higher levels you have to up your game as a GM and design more involved encounters. Not only do you need to do this to keep the pressure on the PCs, you need to do this to keep experienced players from being bored. There is only so many times that you can be ambushed by monsters who have no apparent motivation on an empty stage with no terrain, who are attacking you by trying to close the range, get into melee and hope to survive to get in a few hits, before as a player you are completely bored. You might get through one campaign with a diet of that, but by campaign two you are going to need to improve your encounter design and add in weather, terrain, tactical variations, encounters with combined arms teams, three sided encounters, complex multi-stage encounters, encounters where either the PCs or the NPCs have particular goals in addition to eliminating the other side, and so forth.
Beyond changing tactics, you can also change what you use as a mook. In earlier editions I was always a big fan of mephits because they were pretty weak but they all tended to have level invariant attacks, ranged attacks, and flight - a combination that is perfect for a mook at higher levels.
As for the first solution that has occurred to you, it's not bad but I would suggest instead that you create a feat that encapsulates the idea of mooks swarming around less numerous foes, and then assume their are 'elite goblin light infantry' that have additional HD and this feat. So for example, I have a feat I often give to certain foes that doubles any flanking bonus - wolves for example have this feat as a racial bonus feat, and goblin infantry often has the feat. There is also a homebrew feat in my game called 'Pack Tactics' that gives you a bonus to hit and AC for each adjacent ally that is also adjacent to your target. For 5e, with its slightly different feat design, I'd probably combine the two ideas into a single 'Pack Tactics' feat. So at high levels you'd be encountering not just jungle goblins, but elite jungle goblin infantry with higher Dex, an extra HD, and the 'Pack Tactics' feat that gave them double the normal bonus for flanking plus a +1 bonus to AC and To Hit for each ally adjacent to them which was also adjacent to the enemy and which also had the 'Pack Tactics' feat. Then I'd design the encounter so that their was a covered spiked pit trap between the PC's and the obvious line of approach to the main body of forest goblins, and where 1 in 4 of the forest goblins also had a vial of flaming oil, and where several forest goblins had vantage points up in trees (with partial cover and partial concealment) from where they could snipe at the party with poisoned arrows, and where the forest goblins were supported by a low level spell caster such as a cleric or a bard. Or I'd have the jungle goblins try to throw weighted nets on the PC's, and have them set up to brachiate through the trees to engage in running battles where they took the high ground and had platforms with rocks on them that they'd throw down at PCs or containers of hornets that would break into mini-swarms, or vials of vegetable oil they'd break against tree boles to grease any player attempting to climb up to them.
Or have encounters like that in the dark during a thunderstorm with a flash flood going on.
Finally, don't let the dice ruin your game. If you ever roll a random encounter that you can't figure out how to make interesting, ignore it or select a different one.
