Round 22:
5 The Royal Jellies VS 20 Deep Elf Master Archers
Note: Yeah, twenty guys with rapid fire bows. Anyway, the royal jellies also have good amount of health, absorb HP when eating up the arrows that drop after hitting them (important note, that HP regained? Nothing compared to the amount taken off) and may split when hit, meaning many, many wimpy jellies to fight.
Round 23:
Pit Fiend VS Ice Fiend
Note: So yeah, take the regular fiend and power him up into a much slower, tanker, but generally better pit fiend. And now you've finally got something on par of the ice fiend.
Round 24:
100 Newts VS 100 Rats
Note: Ah, yeah, big numbers.
EDIT:
Uniques are monsters with names. Sigmund is a unique, as are the hell lords. testspawner isn't even in the game, it's just part of the arena where you can summon infinite numbers of monsters to fight forever. It's not even a real monster and from what I know, it's invincible.