Why? Because you're going to have to fight that same battle another five times in a row. As the fight draws to a close, use the opportunity of only having one enemy left to use items to get your party's HP and MP back up.
As always with these harder battle, focus on destroying one enemy at a time. You'll need to make sure you're healing when you need to and not taking risks, as well as using any support powers Mike has to increase the abilities of the other party members: Strength for Johnny and Francis, Spirit for Nigel, etc. The army of monsters you fight is pretty tough if you don't take them seriously. The game expects you to lose here, and it's one of only two places apart from the arenas where the game continues when you do lose. When this is over, he will unleash his army upon you. DO NOT overwrite this save until you've unlocked the next achievement! Now, head E again for a scene, and some dialogue with a character called Richard who's aboard an airship flying above you. Go E, and head through the pipe, then SAVE YOUR GAME. SAVE when I tell you to, or face another play through to this point! IMPORTANT! There are two mutually exclusive achievements during this chapter.
