Millennium - A New Hope

"Guts" is a passive ability that only Marine possesses. Guts will activate when Marine is KO'd. She will automatically revive back into the battle after a few turns. Simply have Marine killed in battle five times and revive 5 times with Guts to get this achievement.

There are enemies in the Ancient Temple that have an instant KO attack that may help you with this if you haven't been having much trouble with the game, if playing on Easy or Normal.