Lias Issodon (FW Raptors Chapter Master) is one of my favorite characters in the game and, I think, one of the best on the table-top. Among his many abilities, he has the WL Trait “Master of Ambush,” which allows him to infiltrate himself and 3 non-vehicle units.
Now, combine this with an allied GK Nemesis Strike Force and you have quite a potent potential strategy.
1 – Pre-game, Infiltrate Lias with a Squad of 4-6 Grav Cannon Centurions, flanked by two squads of 10 Purifiers, each equipped with 4 Psycannons and Halberds/Hammers to taste. If you can put Lias and the Centurions into cover, this his natural Shrouded USR means they are rocking a 3+ cover save in addition to their T5 2W and 2+ armor.
3 – On top of this, if the Grey Knights have anything Deep Striking in turn 1 (due to Nemesis Strike Force special rules), then they can land within 6″ of Lias and have zero scatter, thanks to his Locator Beacon. Add in a few “shunting” Dreadknights and you can quicly overwhelm an opponent’s battle-line with target saturation that should overwhelm his targeting priorities pretty quickly.