That’s the one thing that psykers don’t really have anything for. You can still do it ofc but it’s a bit finnicky. None match a hammer or an oggy / vet with a dedicated boss killer ofc. But here’s a few ways I know of to still get pretty good results:
While it prolly qualifies as a gunpsyker, your basic right side crit, Scrier’s & Disrupt Destiny build with Laspistol does really seriously good weakspot dmg on anything. So unless it’s a Chaosspawn (bc good luck getting headshots on that thing unless someone keeps staggering it) I can usually get ~30k+ dmg in per boss if there aren’t any actual bosskillers like hammer zellies around.
Next best thing would prolly be a crit Voidstrike with Surge and Nexus, perked for +unyielding ofc and again aiming for those headshots. While this has lower dmg than the one above, if you have your buffs stacked and/or get Surge procs especially if your team participates, you can keep staggering the bosses. The main trick besides headshots here is to not waste those True Aim procs when the boss is still recovering from the previous stagger, but wait for it to recover and then stagger it again. If you pull it off, you can keep them staggered indefinitely. Ofc waiting for the recovery means there’s always a chance the boss will just instantly turn around to chase someone else instead, and bye bye headshots & stagger.
Purge is surprisingly good if you have BB, especially with Vent. While you won’t be doing big bursts, the Soulblaze does deceivingly respectable dmg and you won’t depend on weakspots for it either:
- Cap the SB DoT on it
- Use the next 10s before SB starts decaying to BB or melee. While Deimos / Duelling Sword heavy headshots are likely better, BB always hits weakspots and has range. Regardless, you still need to use BB to keep your peril up for Warp Rider, which benefits all of your dmg including the DoT.
- Switch to Purge to puff it within 10s to refresh the DoT, and repeat these as needed.
- Vent with Creeping Flames at 80%+ peril adds another 6 stacks of SB on top for 10s and refreshes the DoT, increasing the DoT dmg further by around 33% while ofc letting you do more BB. It also doubles as a horde & sniper killer, making it easier to focus on the boss. But remember that Purge can’t refresh stacks above its own cap, so after 10s the SB stacks will fall back down to 15.
- Ofc there’s some more layers to this if you really wanna minmax, what with keeping Perfect Timing running for that +15% warp damage too.
- Remember that Purge is a crit staff because crits apply 2 stacks of SB and SB has a logarithmic dmg curve (each stack doubles its dmg in the start, tapering down the higher those stacks go). So you want at least Nexus on it. Also remember that SB uses your equipped weapon’s stats so +unyielding on purge won’t do you much good if most of the time you’re using a melee weapon with no such perk.
- When I tested this in psykhanium with a full build but without Perfect Timing (bc the reaper died too quickly if I had it), 15 SB stacks did ~300 dmg/s, where after Vent’s 21 SB that went to ~400/s. Because of the way dmg is calculated Perfect Timing actually added more than 15% to that and even at 15 stacks it did around 380 dmg/s iirc. So if we average this at 400/s, that’s 12 000 in 30 seconds passively from the DoT alone that you barely even have to bother with, otherwise doing your normal dmg on top.
- In practice on T5+ pubs, you don’t always get to fight a boss alone since there’s enemies everywhere. Similarly, your team might be short a few members or is struggling with other enemies instead of being able to focus on the boss. While Purge can effortlessly burn bosses and adds all at the same time, that DoT is going to matter more the longer that fight takes. So if the fight takes a minute while you’re focusing on burning adds or even just kiting and surviving, SB on its own will now have done around half its total hp in dmg (iirc they have around 45-50k hp on T5 now, at least according to Scoreboard).
Purge in general is very underappreciated and extremely strong if built and used right. But I think I’ve blabbed enough about Purge. 
Finally your typical Trauma with +25% unyielding, Flurry and for example Nexus again (the latter blessing doesn’t make much of a diff tbh, Rending Shockwave is better for the team ofc) does a LOT of damage against the slug boss. So any typical trauma build which tbh is best with +flak & +unyielding and Flurry anyway, is surprisingly good against that one. It’s decent against the other bosses too but nothing remarkable.
Edit: While I said finally, I forgot one thing. The staff primary fire is surprisingly strong esp on weakspots and has that fire > quell > fire trick that can be macroed, and seriously increases its rate of fire. So theoretically if you had a great staff with Transfer Peril & either crit or Surge / Blazing Spirit (for the DoT again) with True Aim, a really good aim, and tons of luck, you could keep blasting rapid fire shots into a bosses’ face indefinitely and it would probably do quite a bit dmg.