I run DH, and one of the missions I had my Acolytes run was to hunt down and capture if possible a rogue psyker (level 6 pyromancer who dabbled in telepathy) and they promptly spent the next four 5-hour sessions tracking down every lead and whisper of the guy I could throw their way. During this time, three of my five acolytes levelled up, including the Psyker, so I was beginning to think the psyker they were hunting wouldn't be much of a challenge for them.
They hunted the guy down, requested a squad of inquisitorial stormtroopers and were rewarded with a kill-squad of 5. The acolytes and the stormtroopers proceeded to clear out a lower-hive hab block, packed to the rafters with cultists and very low level psykers. The mission went with almost no hitches, although the kill-squad fell prey to a cleverly concealed (if I do say so myself) heavy stubber, which also critically wounded one of my two guardsmen. They left him in control of the heavy stubber and proceeded to search the building for the rogue psyker, who turned out to be in the tunnels beneath the block. They cornered him in a collapsed sewer, and he started casting fireballs and firing his auto pistol trying to keep them at bay, until he had a power blackout and they started advancing on him that is. He managed to hold them off without taking any serious damage until his powers returned, at which point my second guardsman decided to charge the psyker.
Now the guy was running flat out, sprinting to close the distance when mr rogue decided to cast flame wall, which promptly erupted from the floor and incinerated the sprinting guardsman. Now everyone else got a bit pissed off and renewed thier assault on the rogue. Emboldened by his kill, the rogue decided to take the offensive and tried to dominate my psyker. One willpower test later and suddenly my acolytes were beset on both sides my magical fire and gunfire!
Now as you can imagine, my psyker wasn't very impressed at having been dominated by a rogue, and in the following turn, after taking control of his body back, he unleashed his favourite power, holocaust. And I think we all know how that ended. (not very well for the rogue XD)