When I play arena, the final level consists of mecha knights followed by a fiend or a moth. Problem is that is all I ever see, the mechaknights are tough don't get me wrong. But something equally as challenging with a bit more variety couldn't hurt.
I suggest putting really strong creatures instead of a band of knights to fight. Something like-
1. Fully grown lichen colonies (at least 4 or so)
2. Alpha Wolvers (maybe a bit less than the knights but still alot)
3. A multitude of different gremlins (One of their best qualities is their variety)
4. Already angry devilites (throw in like 3 trojans just in case)

Those are some suggestions that I think would not only make a variety of fights but puts more weapons in the mix.
The only time I can use a pierce weapon is charging into a giant group of knights, trying to cut up the healing moth while screaming as the knights dice me to bits.