Just want to clear something up. I am not the GM of this particular campaign. However, of our group I have the most experience with Tabletop RPGing (having run at least a few D&D campaign-- different thing I know), and so my GM often discusses these matters with me, and seems to value my input for how he can shore these things up. We've had very long discussions over how to balance out my Psyker to keep him from storming ahead of the rest of the party, despite the fact I've declared my intent to have him be a Templar Calix.
Right now the Assassin just happens to be the big problem right now.
On the point blank range thing, many of the fights that have broken out have involved either cramped quarters, where everyone is getting point blank range, or the Assassin's been sneaking around and essentially starting combat at that range.
I have problems with this. If "everyone" is getting Point Blank range, then your fights must be in an area smaller than my living room. PBR is 3 metres, that's nothing! In such close quarters, engaging in melee should be trivial (and devastating for your assassin).
To sneak up on a bunch of people is harder than just one, and especially if they're guards "Guarding" the area. Does he sneak up alone, far ahead of the rest of his team? If not, his teammates (which aren't as Sneaky) are likely to alert the guards. If he does, he will practically be alone in melee range of all his foes until backup arrives.
And even if he does sneak up on the baddies, he can only reliably take out 1 guy, before he should be swamped by the enemies.
Know the combat rules. You can't Move and Full Auto on the same turn. After the initial surprise attack, his position is known. If he moves in close, he should be stomped. Return fire, and melee chargers.
Play the opposition like you would play your own character in such a position. Let them use cover and concealment, let them use teamwork and tactics (suppressive fire, flanking, flamers and grenades to reach enemies in cover, etc)
Know your modifiers. Perhaps the guards have some gear to help them with their awareness. Photo-lenses, auspexes, extra training, etc. Have them Assist each other. Have debris and other things scattered on the floor, to make Silent Move more difficult. Chains and charms hanging from the ceiling, making noise as you try to move past/through them.
And why is there convenient cover less than 3 metres from you opponents, anyway?
Have them positioned so that they will have crossfire and be able to see each other. (The palace guards aren't all standing together in a huddle at the main gate, you know. They're spread out, on roofs and balconies, and positioned so they can see each other and any foes trying to sneak up on their mates.). Have a hidden guard or two, spying from out of sight. With a sniper rifle.
I dunno. Perhaps an example would help?
Sure. I can give you two.
Quick note on context: Our characters investigation has recently tied in with a rebellion on a Hive World. During the events, our characters ended up having to infiltrate through Rebel lines, and make our way through the ruined Hive City the rebels are using as a base, to try and locate the leaders of the rebellion, and get some answers out of them.
We'd been working our way through the Hive, had had more than a couple close scrapes, including the one with the fortification I mentioned above, and managed to find some workers building a barricade around a more central military installation. They were protected by three guards, or, at the very least, the guards were protecting a supply truck. We all basically sat back and waited (well. My Psyker waited, one of our guardsmen spent the rest of this time setting up a sniper shot), while the Assassin weaved through the rubble of the destroyed buildings and used both the ruins of the Hive City and the truck itself to cover her approach. The GM went ahead and took the opposed tests, but the guards noticed absolutely nothing. Then of course, the Assassin got into point blank range behind the guards, and just went ahead and opened fire at point blank range. The Guards were surprised, and thus, counted as Stunned for the round, allowing the Assassin to turn two of them into bloody chunks (first when she opened fire, then on her second round, while the other two guards couldn't react due to being stunned). The third guard was pelted by the sniper shot that the Guardsman had set up, drew a las carbine and took a shot at the Assassin which missed, and the Assassin then finished him off on her next round. The workers themselves were unarmed and spent that time fleeing. We managed to catch one and interrogated him, and then got out of there before reinforcements could shot up.
In the second example, we had finally located one of the two leaders of the Rebellion (admittedly the lesser of the two leaders), and made our way into his hideout. He made a successful awareness test while our characters were checking his room for any plans and information regarding several of the events that had happened prior to this (including the massacre of several high ranking noble families and the assassination of the planetary governor), as well as information on where the rebels were being funded from, etc. Anyways, he noticed our party, and demanded we come out. The character in question was flanked by two guards, one with a Shotgun the other with a lasgun. He himself was packing a hand cannon and chainsword-- considerably beyond our character's current gear.
He tried to get our characters to disarm themselves, and the Assassin made a successful Deceive test, to seem like she was offering her autogun to be taken from her, prompting one of the Guards (the one with the lasgun) to walk over to take it from her. Then she opened fire point blank on him, catching him by surprise and turning him into bloody chunks. This was, incidentally, an "elite" guard the GM had prepared with higher than average statistics, and better armor than the other ones we'd been fighting, which prompted him to complain about it. The Captain ended up being charged into close combat by one of the Guardsmen, and the other Guardsman charged the guy with the shotgun, essentially preventing the Assassin from being effectively attacked, and allowing her to spam her full-auto strategy throughout the whole battle. Granted it was into Close quarters, but the point blank thing still allowed her to perform just fine (sadly we failed to capture and interrogate the commander, as he chose to put a handcannon bullet through his own head, when he realized he couldn't win, rather than be captured by us).