As a GK player, I've always had an easy time dealing with Mephiston. Feed him a sacrificial unit, then counter assault (because even in objective games you should never spread out more than necessary, and even then only at the last minute). Plus, half the time Mephiston's gotten into assault with a full 10 GKSS I've managed to kill him with just them. And if my opponent dedicates a RAS to the assault as well, that's just more easy prey for me. 10 GKT plus a Grand Master with grenades will kill almost anything on the charge.