Expanding on Patricia's idea, what about (partial) invisibility for several monsters? The Uber-Heresiarch (and maybe regular Heresiarchs) could have an invisibility spell they could cast at tight moments. It could also be cast by Ettin Shamans or whatever.

As I said befor it should be easy to add. I've added it to my KH to do list (its getting quite long) and I'll do after I've to those damage skins in. What old enemys do you think should have this? Should it be an ability for possesion? how transparent should I make them?

Are you already working with the SE source from CVS or not yet? I'd say give it to the Heresiarch with a length of about 10 seconds and a cooldown of 1 minute or so, then we'll see how it works. Regarding transparency, it should be nearly impossible to spot if before a similar-colored background (how mach is that? 5%? 10%?).