Well, It wouldn't really be a 'Battle Cry'... More like a slow, quiet laugh, with the barest trace of a grin, as I prepare my Chainsword, The laugh getting louder, and the grin getting bigger as the battle progresses, Until my enemies either get the quite massive hint, and start running as if their lives, bodies, and souls depended on It, Or I end up in the middle of the battlefield surrounded by my enemies bodies laughing my head off.
Some decent music to use while employing this strategy: http://www.youtube.com/watch?v=ZYIkCb3ADOU
Failing that...
"Run, Run, Before your well done!"
Theme:http://www.youtube.com/watch?v=FYnzpPdsHRQ&feature=related