So to get this achievement you need to complete boss hunt on craaazy difficulty.
You need to do normal and hard first. (or have an friend with the craaazy bait)
So the easiest one is the gnomus brothers. (Wich is the one i have done it on)
Do this with friends. I did it with randoms and i won. But you need much luck. (One even left midway and we still won)
The best characters for this are:
2x toxic brains (could be swapped out for electro brains or even normal super brains)
2x astronauts (or other scientists with enough range and damage.)
How to do this:
Place the craazy bait.
If anybody gets hit by an special bomb gnome they go to the rome area to the statue.
Survive until the boss comes.
Scientist and brains stick together and the 2 groups go the other side. Shoot down gnomes on the way.
Shoot on the blocks until the color of the gnome is there (if the gnome is blue you shoot the block until the colour is blue etc.)
Do this until all blocks are the same colour of the gnome.
Then you need to shoot down the gnome, if the timer ends the gnome changes to another gnome and anybody who is cursed wil die. If he dies you need to banish him. An super brains needs to do this. When he is banished another gnome comes.
Repeat this another 3 times.
Scientist tip: when all teammates are dead just teleport around the map, it worked for me. Don't bother to revive.
When you complete this you get the achievement and an hat.