The more that I think on it, the more a rock solide squad of Chaos Space Marines would probably cut it for me. Just take the boys with me, and bring down the pain!

An aspiring psyker of some kind could use a few sycophantic servants/apprentices. Low-cost and disposable. Use that "Betrayer" Talent and feed the f**kers to the Warp if you get a crappy Perils of The Warp (or equivalent) roll. Saves you the Fate Points.

A female Slaanesh cultist. Try to use her as a info gather.

How about a single daemonic soldier of a common variety. Nothing more, nothing less.

Oh no I lie. If you damage this daemon, say chop it's arm off. The arm regenerates in midair, the body regenerates. Now you have two. Each strike that doesn't totally destroy the beast through explosives or faith will cause it to multiply.

Strike him a few times to create a decent horde and send them after a Death Watch kill-team. Wait for their numbers to grow before they overwhelm the silly Loyalists.