I think this group has it the wrong way round, if the Inquisitor tells you he has a delicate job for you, you do it delicately and if you screw up its on your head not his. He is the one that can just tell the starship to bombard your position if things get out of hand not the other way around.
It does seem like they have covered almost all the bases, just make sure they dont get access to Hexagrammic Wards however nicely they ask. I hate Fearful Aura, it just makes a mockery of most humans, the only good thing was when I got to use it on the player who had previously been playing the Psyker. After he abused it he could hardly complain.
I think what you need is a Master Sorcerer with a very high WP bonus and some Telepathic Powers. Zone of Compulsion would be a good one if you could get your group in range and give the order "attack your teamates". Or just dominate the Biomancer when he runs out to do his thing,he shouts the all clear they all stand up to fight and he uses the power with max dice, as all of your NPC's cover thier eyes for the actual attempt. Lets see how they like fear 4. Also chuck in my favourite Malefic Curse and blind anyone who is annoying you, perhaps even perminantly, if he has a high BS then Hellish blast ignores armour so is great for getting past those pesky tank types. Daemon summoning rituals, if your are feeling bored get a squad of bloodletters in there lets see how they like fighting something with a higher soak than them.
If they want to be a kill team show them what the Ordo Malleus has to kill. Or if they are Ordo Xenos chuck them into the middle of a fully fledged genestealer cult. Of course if they are Hereticus then you can use the insanely powerful psykers of doom. That little boy who is a psyker that is being shielded by his parents, not quite actually a 350 year old Beta class psyker that hasnt aged and is reanimating two corpses to be "mommy and daddy". The warp is bleeding through and the Daemons are circling, a psynicience test says, "DONT USE BLOODY PSYCHIC POWERS IN HERE, ITS A TRAP".
Hope thats some ideas.
Kaihlik
EDIT: Of course they are getting pretty brutal, maybe another Inquisitor is looking to deeply into thier activities and does not like what he see's, maybe he suspects them of corruption. Torture is always a last resort even for the Inquisition (since most info gained from torture is false), doing it for fun smacks of something tainted. In order to save face your Inquisitor declares them Excommunicus Traitorus. See how they like fighting the Inquisition.