Build a psyker as normal. At rank 4, grab Divine Shot. At this point, you should have the cash to get a missile launch and some missiles.
Shoot tough enemies with krak missile flawlessly. Shoot groups of enemies with frag missiles flawlessly. Be this cool: http://i.imgur.com/g1CB9L8.jpg
I thoroughly enjoy the idea of rocketing guy in the face from 800 meters. I imagine just sort of pointing the launcher into the air, completely at random, and firing. A trail of comically whirling gray, puffy smoke follows the missile as it follows a completely non-ballistic trajectory before striking the target with unerring precision.
Now, someone might point out, "but FlailBot, psykers don't get Heavy Weapon Training (Launchers)" and I'd appreciate their excellent capitalization and then say, "Yes, but Divine Shot is total bollocks and that's irrelevant."
Comments? Criticism? You'll find Divine Shot on page 171 of the Core rulebook.