Given the depth of the "grave" and the nominal "bulge" upon the front face (later downward after its 90 degree rotation), I figured there was a substantial "gap" within the "tomb". and the way we saw Mitchell stumble and collapse back into the grave just as the slab descends, it's quite possible there was minimal trauma to his body.
But I figure something had to happen to keep him from levitating (or dramatically exploding) the slab in a gesture of god-like fury. Maybe he raised his head high enough to have the bulge of the slab strike it, either killing him instantly or knocking him unconscious long enough to die of asphixiation once the slab "sealed' the grave.
"But the phaser rifle didn't hurt him," I imagine you stating. I used to wonder that too. Maybe because he was aware of the impending phaser shot Mitchell was able to use his powers to "shield" himself. But with the slab falling, he was caught off guard and his body was still prone to injury if he didn't use his abilities.