You are playing with the "Allow monsters hostile to each other" option turned off. I added a check so that they will use spells in the monster arena.

BTW, who wins:
[1] Serpent vs Unicorn? Serpent.
[2] Serpent vs Babble? Babble.
[3] Serpent vs GWoP? Serpent (Not even remotely close)
[4] Serpent vs Talos? 50/50.
[5] Serpent, Unicorn, Nodens and Talos? Talos every time
[6] Serpent vs Zombified Serpent? Zombie, of course!

Still, monster summoning is turned off so its not truly fair ...