it's make it or break it for a lot of people
my issue is more with the poor writing at its beginning than anything else
Did you primarily attack with powers or with weapons? Doing both isn't exactly advisable since, you know, one attack per turn for most people. If you're going for buffing yourself with psyker abilities, melee is the best route. Biomancy can boost your stats to ridiculous levels, let you poison enemies, and also act as support with healing. Pyromancy lets you light yourself on fire and make it permanent so you can deal fire damage with every attack, like an absolute madman. Also cause your enemy damage for even attacking you with Burning Blood, which is triggered by the fire you can set on yourself or other DoTs through items. Sanctic is the best if you invest to the point you get Sword of Faith and have a Master Tactician to linchpin your resolve into the hundreds.
Telepathy and Divination are more for pure psykers. It might seem like a lot but some abilities are either niche or just not worth it. Compare Dominate to Sensory Deprivation: one brings enemies towards you but they can still attack while the other does this
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A pure psyker investing only in psychic abilities, the minimal amount of class talents, and most of his common talents into warp talents will bully the absolute fuck out of enemies in the mid to late game. Most of my problems were caused by positioning and occasionally perils of the warp from so many abilities in one turn before Cassia can clean up my mess.