I've been running RPGs for eight years, and I almost always am in some way involved in a game of Dark Heresy at any one point, usually GMing it.
I've hit a rough spot with my current party though, they're a fairly high powered bunch and they tick all the boxes. They roleplay well, they could tell you what's happening in the plot if you asked them, they remember their Inquisitors name half the time, they really want to get the bad guy.
Only problem is their composition. It's a three player group, Pyromancer Psyker, Untouchable and Cleric. The Untouchable is something of a swiss-army knife and has a little bit of everything and lucky enough rolls to get away with it half the time. The Psyker is something ridiculous like Psy Level Eight and fifty Corruption points. He's also undergone a Cybernetic resurrection after he blew himself up by not realising that Inferno has an area of effect and, yes, he was in it. He now has the Machine Trait, Fear 2 and is a walking talking Ravenor with none of the subtlety. The Cleric would be the groups face, but the group scares the bejeesus out of anyone they meet. One is a soulless abomination and the other makes people wet the bed. This isn't the problem however.
The Problem is combat. The Group are investigating Slaaneshi taint in the Adeptus Mechanicus (It runs all the way up to the Grand Fabricator of the sector, and has suffused the Legio Cybernettica division) and they further the plot by banging people over the head with pistol butts or ripping the information from their mind. I don't mind that, that's the type of game they want to play, but it's hard to challenge the players with interesting and engaging combats so they feel like they earnt this information.
There's pretty much nothing that the Psyker won't respond with that isn't Molten Beam. If it had a shorter range, I'd be fine with it but it's currently a forty metre one-hit kill on whatever is the biggest threat. But if he's damaged or incapacitated, it's a wipe for the party as the Untouchable has a boltgun and a sniper rifle that he always misses with when it counts most. The Cleric isn't built to deal with combat and is trying to build a small army of fanatics to follow him about with flamethrowers and pitchforks to work around this.
The Group have made a fairly obvious enemy of the Mechanicum, surviving two Ambushes staged through the power of Molten Beam and Sniper fire. I'm toying with the idea of high end elite Assassins, specifically tailored to deal with the PCs. They're the most prestigious and credible threat to the Mechanicum in the sector so I don't think it's unreasonable, but what do you guys think about making NPCs specifically to try and kill the PCs?