This achievement is actually very easy with a reasonably high level Will user.
Arrange your spells like this...
Level 1, 2 & 5: Time Control
Level 3 & 4: Inferno
The Time Control spells slow the main game clock as well as the action so you want to spend the entire fight in slo-mo.
The level 1 & 2 Time Control allows you a quick cast so you can charge up the full level 5 spell.
Once everything grinds to a halt run to the middle of the enemies and unleash your level 3 or 4 Inferno (indirect only) spells to incenerate everything. If there's any left over that are out of range use a decent ranged weapon ("Red Dragon" pistol).
Keep a look out for the flitswitches and grab those as quick as you can. For the red ones just use your Inferno (indirect only) when close enough, you'll get the switch and a few kills. You should look to get around a 9x multiplier but you may as well aim to pick up the 10x achievement.
If you're worried about health just add the "Ghoul Augment" to your favourite weapons. This will drain the life from enemies and add it to your own.
This tactic works for all rounds with one tiny exception: The Banshee. Time your Inferno spell right to take out all minions, then use a directed Inferno to inflict the most damage to the Banshee. A few shots from a decent range weapon should finish it off before the second set of minions.
I've only played this level twice and got c26,000 (9x) and c31,000 (10x) with NO chickens. Add a chicken suit and kick a few of them and the score may be higher, I'll confirm this when I've done it.