In the campaign I'm running I was planning on having the kill team confront a wayward Ordo Xenos inquisitor. I wanted to have him loose some arco-flagellants on the kill team prior to them having to fight the inquisitor himself. I remember from the game Inquisitor that the arco-flagellant was one of the few things in that game with a chance of taking down a space marine (assuming it could close), however there are no rules I know of for such creatures in Deathwatch or in Dark Heresy. Also, when I thought about putting the rules together myself, I noticed that the iconic weapon of the arco-flagellant, and their namesake; the electro-flail, is utterly terrible and would be of absolutely no use against a space marine, still, I'm fairly sure arco-flagellants are meant to be pretty scary.
Basically, does anybody know of any rules for anything like an arco-flagellent/has anybody put together any unofficial rules and what would be your suggestion about making them more powerful so the battle brothers don't brush them aside. I don't want to field them as a horde as I feel that dilutes how scary they are meant to be, I'm really looking to make them about as powerful as their Inquisitor iteration.