There is a cheat in the melee section of SC3. Pressing ALT-F9 during a battle starts it over from the beginning, restoring the health of you AND your enemy and removing the planet (if there was one) in the process. There is a bug in SC3 that is to your advantage, kill an enemy and hit
ALT-F9 while he's exploding.

The battle will restart, your health and YOURS ONLY will be filled back up, and your enemy will die immediately.