I think her being slow is OK, if she went first, none of her special abilities would be useful. Instead charging her attack on death and finishing blow are very useful if she is last to go. Plus her leader skill can speed up Daka, who can wreck the enemy with her stuns.
AI, as mentioned, is atrocious, and definitely liability on arena defense.
I do agree that Rampage not triggering on her own kills is a bug, as is party heal and inability to cleanse all buffs.