This demo shows how I defeated the level 12 boss, on ACE difficulty. I must say that it looks deceptively simple, because the the red room has already been cleared of all minions without using the cloak inside the secret recharge room nor the invulnerability hidden above the guide-bot's starting cage.

The demo starts just outside the red room, where I have dropped off some ordnance in case I needed it. (Turns out I didn't.) I enter the room and immediately go the recharge center and grab the cloak. Then I track down the boss and let him have a few Megas. His return fire is impotent as it can't track me.

Having softened up the boss a little, I now go for the jugular -- back out the red door to grab the invulnerability. Wasting no time, I re-enter the red room and search out Mr. Big. As soon as I find him, I treat him to a Gauss and Mega feast. Before my invulnerability wears out, I can back up and enjoy the fruits of my labor! :-)