I would rather the XP be based on the ratio of damage plus a little extra to the guy who actually takes out the enemy. If the system can be more elaborate, taking into account the difficulty of pulling of a particular attack, combat text acknowledging the improbable victory would be nice:
Thrasher scoops up and flings a rock at the Mega-Laser Spiderbot. It shoots directly into the tiny heat regulation exhaust port between the armored plate seams at its thorax, clatters through the entire length and into the robot's core, causing a complete system overheat that results in a spectacular explosion. Sgt. Ace is caught in the blowback for 10 CON. He is unconscious and near death. Thrasher gets 200 XP.