How do I run an arena / waves of enemies encounter

[NihilsticBanana]

I want to throw my players into a big arena, I figure it would be fun or cool to put them through 5 waves of enemies slowly scaling in danger or difficulty so, for example, a decently sized troop of goblins for starters and then slowly increasing the CR of the monsters until finishing off with one big CR monster. Thing is I'm not to sure how to balance this... What I mean is how do I handle the CR of this? Do I hit them with things under their CR and slowly scale up to something above? Wouldn't they like.. be in super rough shape by then? Should I let them get all healed up after every encounter? Should I throw them some NPCs to take some hits for them? Help me figure out how to handle something like this the best way possible, please. Thanks in advance.

[Ikorus]

Give them one or two free rounds between waves as the next is being put into position. A round is 6 seconds, so try to figure out how long that would take and go from there.

If they can manage to heal to full? Good for them. If not, oh well guess you gotta do some in-combat triage. The whole point of something like this is to wear down their resources for the day a bit before dropping the hammer, not to actually try to kill them. Spell slots, items, special abilities, anything they use up is a win for setting up an actual challenge down the line.

[Quixote]

I'd second that idea: I'd say 1d3 rounds would be good. Enough to give them some breathing room without making things too hectic or too comfortable.

I'd probably start with a CR 2 lower than their APL and finish with something 2-3 higher. Goblins, two ogres, some lions or tigers, orcs on dire wolves, then a trio of hardened gladiators--something like that.

Two things I would strongly suggest:

1. Avoid a single foe in any one combat. Not just in this encounter, either. A lone monster just can't provide a very interesting combat against a group.

2. "Arena" does not mean "big open space with no terrain features". Pits and spear traps, walls of fire, rising/sinking platforms, shifting floors, etc. Each wave can be accompanied with a change in the arena setup. The party might even be able to guess what sort of threat they're about to face from the changes or come up with a hasty strategy.

[Ikorus]

Agreed about the no solo boss. If its significantly stronger than the party, it wont be killed too fast, but then you have the opposite problem, playing the monster properly without slaughtering the party will be difficult.

You see this problem with dragons a lot. Dragons played right, even young ones, should not be f~%!ed with lightly. You need to be intensely aware of what you're getting into, and plan and prepare accordingly to make the fight as unfairly stacked in your favor as possible or you're all gonna die.

In an arena setting like this, you cant KNOW what to prepare for, so you shouldnt use that type of challenge (the single extremely powerful in general, not dragons specifically).

What might be fun is to pit them against a mirror. Another adventuring party who are the reigning champs. 3-4 humanoids with decent enough stats your caster/s cant trivialize it, but weak enough individually that they wont completely rock your party.