You need to be weaving in and out between your teammates; your strength is in the utility you provide as the delegated support. This more applies to Heresy-Damnation where you’re pretty much Gimping yourself by using anything other than the Surge Staff (more so Damnation).
Never full charge the staff. 1/4–1/3 is enough. Spam it around, distribute that stun any where it needs to be. Don’t overfocus the same targets. Get a feel for when your F is best used. I run the Soulblaze Feat on 30 so in case of Hordes, along with the passive warp stacks, I can aid in melting waves with it.
Purge for 2 or 3 seconds around 70-90ish% Peril with your Staff. Spam your stun. You’re not the killer, you’re the one that saves your team’s asses.
Never be static (still), as said. You always want to be in the middle of your teammates, but spatially aware. Get used to abusing your dodge sensibly to supplement movement. Get used to looking over your shoulder.
Get used to quickly tabbing to your melee to quickly dispatch of trash. If you HAVE to melee, then you have to focus on that first and foremost. Cause if you go down, that hurts the team – a drop in any coherency is bad. You, as the Psyker, are made of Papier-mâché.
There is only so much you can do in your given role. As long as you’re doing your part, the rest is trusting your team to have synergy with it.