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.
Summary:
Hit ALT-F9 while an enemy is dying to restore your
health.