Quote Originally Posted by Paradigmatic View Post
To be specific: you kill Attila in battle once during a single turn, his campaign avatar will retreat. If you kill him again during the same turn (not sure if it makes a difference, but that's how it happened to me), you get a message about Attila fearing defeat more than death and his campaign avatar disappears like a wounded general/agent. He eventually respawns out of sight (can be a few turns I think, like a wounded general/agent). If you kill him again after that, he's permanently dead.

Maybe the number of times he has to be wounded varies with difficulty level? I can't say I paid attention if this happened differently in Hard and Normal. But I'd say, off the top of my head, no.
I've heard everything from two to seven times but I suspect the truth is you have to annihilate his army twice, which would likely mean "killing" him up to four times.