BenjaminPey
Adventurer
Sure. The "all the monsters in one go" is probably the least efficient way to use this budget. But as few as two waves in the same big battle could be enough for a thrilling fight, depending on the particulars.
What if they are all hard+?You are kinda crashing into the limitations of 5e. 6-8 medium to hard encounters is such an epic slog that attrition really is not a thing that encounter design can fix.
As I said, I have been running mostly big set pieces where the party can just let loose. But a player has explicitly asked for more attrition based design. Hence why I am asking.In theory they shouldn't. "High difficulty" is the ceiling for a whole day.
It may obviously vary depending on the players, characters, magic items, etc., but by now (level 15!) you should have a feel for how to tweak the ceiling for this party.
"Bounded accuracy"... The monsters are bounded, the PCs are not & are almost guaranteed to be far in excess of the bounding at those levels.What if they are all hard+?
Thanks.I would think about somehow telling the players that today is not the same as the other days in that there will be several encounters instead of just one big one. Maybe the PCs are given the information, but the NPC says that there are several other layers of defense to fight through before you get to the big guy. This at least lets the players know that you are not trying to play gotcha all of a sudden in game style.
A ruler of a kingdom being threatened would try to bluff and secretly take out the threat like others mentioned. They would also not be afraid to send the army to attack the PCs lands and split their attention like some sort of Sun Tzu stuff. Bad guys might take family members to force the PCs or (friendly NPCs) to do things to hurt the PCs. This also makes the players make decisions about the henchman who is now betraying them. Which can lead to clues about how to strike back or to make a double agent now rather than just kill them.
I also found that I give out too many small things like potions and such that everyone forgets about. I now do not worry about what numbers I throw at PCs at this level. I will not try to tailor the threats to the party and have only one fighter type and two caster types since the party only has this and now just throw 10 low level clerics that can cast hold person or 100 soldiers that might get wiped with a fireball or a teleporting blink dogs that are demon possessed to gain surprise. The players will react and the sheet has something they will use.
As I asked before, how does limiting it to only Hard encounters affect that number, if at all?You probably aren’t going to want to hear this, but… 6-8 Medium or Hard encounters between long rests will achieve the effect your players are looking for.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.