I actually find greaver arenas much easier now-a-days. Why? Stand in the center of the final wave, have all 12 greavers rush you in the center... And throw a stun vial once they're all there. You stun them all (fiends are weak to stun), flinch their attack, and then have free reign to punch a bunch of damage into them all at once especially if you have a heavy sword or bomb.

But yeah, that final wave should only spawn once literally every other wave is completed, so you should only be bum-rushed by 12 elite monsters. Here's a handy list on all monster spawns in the game that I made: https://i.imgur.com/v8zxHxt.png

It should be noted that while some monsters in the "special" area may not seem special, they only have predetermined spawns and cannot spawn in place of any other monster. Additionally, despite the wiki labeling gremlin knockers as "minis", the will only spawn in places where other "grunt" tier monsters spawn - since this is a list based on spawning behaviors, I knockers are classified as grunts.

We really ought to have more variety for certain areas - beasts only have 3 sections covered, and really need to cover more ground. The red outlined sections in the picture indicate what type of monster the damage type is lacking in, and some areas where it may be beneficial to add something more to the game.