I use the Grolar as my assassination jack with Straks. He casts Superiority on it at the start then it lurks near the middle of the line. When the enemy caster is within 20" of the Grolar, Strakov casts overrun on the Grolar or himself and loads it up with focus, then feats. Strakov can then kill a trivial enemy model to provoke Overrun or the Grolar can charge a trivial enemy model within 7" of the enemy caster, crush it with the fist, and overrun to the enemy caster with an initial hammer, autocannon, and 3 focus. Works well and best with killbox scenarios.
Ruin does well too in this role, especially if you can get him souls before his activation and the terrain is pretty open.