It’s possible to recover all of your health before fighting a mecha in the campaign mode by healing yourself with any hp recover drone in the first turn and quitting the battle before enemy’s turn. Repeating this process will eventually get your mecha to full hp at no cost.
Some possible fixes:
add a limit to the quits, being one per battle or 3 in total;
make the game save your current hp before entering the fight, so if you quit after the heal, it would return your original hp (or less, if you get damaged).