This Seeking Tech guide will show you how to get the “Not Half Bad!” trophy and achievement in Metal Slug XX.
This will unlock when you “defeat Rebel Gigant with at least half life (Mission 6’s boss).”
Keep in mind that the trophy will still count even if you died during the fight. You just need to make sure that your life bar is more than 50%. If it gets below that, then wait for the boss to kill you so that you can respawn at full health.
You can use the following video walkthrough as a visual aide: