Hussar
Legend
A common complaint or criticism I see about 5e, particularly at higher levels is the lack of serious challenge in combat encounters. People talk about 5e being on easy mode and that sort of thing and I can see what they're talking about. If you have a group that is really tactically minded and work together well and possibly are well versed in squeezing out bonuses, then, sure, 5e monsters are probably on the weak side. That being said though, there are a number of solutions ranging from very simple to somewhat prep intensive that can ramp up the difficulty of an encounter. I'd like to outline a few options here.
Before I start out though, I want to make some of my own presumptions very clear. In my experience, 5e combats don't take a very long time to resolve - 3 or 4 rounds by and large. And the problem here is that the monsters just don't get enough actions to lay the smack down on the PC's. In a 3 round encounter, a monster might only get two rounds of actions and, well, that just isn't going to make anyone stand up and pay attention particularly if the DM has an off night and rolls a miss or three. So, with that in mind, most of these solutions that I'm proposing are meant to either increase the number of actions per round or increase the number of rounds in the encounter, or possibly both.
1. Monster HP
This is likely the simplest solution. 5e monsters from the Monster Manual are given average HP. A Frost Giant forex (CR 8), has a 138 HP - 6/die and Con bonus. But, that monster has 12d12+60 HP. If you give that monster 204 HP, it's going to live, if not twice as long, at least half again as long as it normally would. So, now that fight lasts 5 or 6 rounds, meaning that the Frost Giant goes from a potential of 150 points of damage (if every attack hits), to 300 points of damage, more than enough to give the players a serious "Oh Crap" moment.
2. Feats
In the DMG, under creating monsters, they ignore feats but, the thing is, adding a feat to a monster, while maybe not 100% RAW, isn't going to radically alter the monster's CR. Maybe add a 10% xp bonus or something like that, but, that's about it. The thing is, feats can significantly change how a monster plays out. You can go with something very simple like the Toughness feat for more HP, or, if you want to be a bit more interesting, things like the Sentinel feat can dramatically alter how the encounter plays out. Imagine the difference of an encounter with 6 hobgoblins and 6 hobgobs, 3 with Sentinel and 3 with Polearm Master.
Personally, I'd go with feats like Polearm master and the like that grant extra attacks. The point here is to ramp up the challenge, and the best way to do that is for the baddies to simply have more actions.
3. Adding PC levels.
Now this one's a bit more work intensive depending on the class. But, again, if we're talking about high level baddies, shouldn't really alter the CR too much. Just make the monster a lot more challenging. I suggest only adding 3 levels of a given class, mostly because I'm too lazy to work out how much adding more levels would actually change the monster, and, in most cases, 3 levels is often enough.
For example, tapping in 3 levels of Bear Totem barbarian grants Resistance to everything but psychic attacks. Boom, your monster now has double HP, effectively, a rage bonus (nice for extra damage) and a movement bonus. Easy to add to the stat block and tons of fun. Or, 3 levels of Rogue as another example. Sneak attack plus that bonus action for mobility and hiding can change the tone of an encounter a whole bunch. Imagine Vrocks with 3 levels of rogue. Sneak attack and fly away to cover every round. Hit and run tactics combined with a battlefield with lots of stuff to move behind. Watch your players sit up and pay attention then.
The nice thing about this is some of the classes are really easy to add. Barbarian, rogue and warlock all fit thematically with a pile of monsters and grant just so many goodies. Drow warlocks that can see perfectly in 120 feet of darkness plus can generate Darkness spells. Whee!
Anyway, there are three options for you to use. Challenging higher level PC's is not easy but, these should help a lot I hope.
Before I start out though, I want to make some of my own presumptions very clear. In my experience, 5e combats don't take a very long time to resolve - 3 or 4 rounds by and large. And the problem here is that the monsters just don't get enough actions to lay the smack down on the PC's. In a 3 round encounter, a monster might only get two rounds of actions and, well, that just isn't going to make anyone stand up and pay attention particularly if the DM has an off night and rolls a miss or three. So, with that in mind, most of these solutions that I'm proposing are meant to either increase the number of actions per round or increase the number of rounds in the encounter, or possibly both.
1. Monster HP
This is likely the simplest solution. 5e monsters from the Monster Manual are given average HP. A Frost Giant forex (CR 8), has a 138 HP - 6/die and Con bonus. But, that monster has 12d12+60 HP. If you give that monster 204 HP, it's going to live, if not twice as long, at least half again as long as it normally would. So, now that fight lasts 5 or 6 rounds, meaning that the Frost Giant goes from a potential of 150 points of damage (if every attack hits), to 300 points of damage, more than enough to give the players a serious "Oh Crap" moment.
2. Feats
In the DMG, under creating monsters, they ignore feats but, the thing is, adding a feat to a monster, while maybe not 100% RAW, isn't going to radically alter the monster's CR. Maybe add a 10% xp bonus or something like that, but, that's about it. The thing is, feats can significantly change how a monster plays out. You can go with something very simple like the Toughness feat for more HP, or, if you want to be a bit more interesting, things like the Sentinel feat can dramatically alter how the encounter plays out. Imagine the difference of an encounter with 6 hobgoblins and 6 hobgobs, 3 with Sentinel and 3 with Polearm Master.
Personally, I'd go with feats like Polearm master and the like that grant extra attacks. The point here is to ramp up the challenge, and the best way to do that is for the baddies to simply have more actions.
3. Adding PC levels.
Now this one's a bit more work intensive depending on the class. But, again, if we're talking about high level baddies, shouldn't really alter the CR too much. Just make the monster a lot more challenging. I suggest only adding 3 levels of a given class, mostly because I'm too lazy to work out how much adding more levels would actually change the monster, and, in most cases, 3 levels is often enough.
For example, tapping in 3 levels of Bear Totem barbarian grants Resistance to everything but psychic attacks. Boom, your monster now has double HP, effectively, a rage bonus (nice for extra damage) and a movement bonus. Easy to add to the stat block and tons of fun. Or, 3 levels of Rogue as another example. Sneak attack plus that bonus action for mobility and hiding can change the tone of an encounter a whole bunch. Imagine Vrocks with 3 levels of rogue. Sneak attack and fly away to cover every round. Hit and run tactics combined with a battlefield with lots of stuff to move behind. Watch your players sit up and pay attention then.
The nice thing about this is some of the classes are really easy to add. Barbarian, rogue and warlock all fit thematically with a pile of monsters and grant just so many goodies. Drow warlocks that can see perfectly in 120 feet of darkness plus can generate Darkness spells. Whee!
Anyway, there are three options for you to use. Challenging higher level PC's is not easy but, these should help a lot I hope.