Harbinger?

[dr. coomer serket]

I've brought the demon head to the end, got nothing.
I've seen stuff saying the rabbit head, but it always ends up despawning during the second boss.

What head goes where?

[Ravo]

All you have to do is have the rabbit's head on screen (and preferably next to you) when you get to Picayune. This means keeping track of the head and not bumping it off screen where it will eventually despawn. Once you get the attack patterns of the bosses down and some DPS you can drop bosses without moving around too much, but you also need a build that you can stay still during waves so as not to lose track of the bunny head. I recommend using low spec mode and lowering the opacity of weapons just to make it easier to see.