Pretty simple, keep and only use these weapons when you get them, play an entire level with only these weapons, which isn't too bad as the Stalker-Pattern Bolter is pretty good - you need to be careful and try to make your shots count to save ammo - but it's really accurate and the chainsword is pretty standard if you have to use it, just make sure not to switch weapons just to be safe.
If you still encounter any problems, try it on easy, it really is super-simple and straightforward, exactly as described, just follow the description and you can't go wrong.