Once completing Act 3, the end of the game (and this achievement) comes fairly quickly. Basically, an end boss must be defeated. Going into the battle, note that the two moveable pillars in the arena need to be up at all times, and that the only place where the boss takes damage is on the TV screen. Also be aware that your D-pad-left time slowing device is your best friend! The boss has five attack modes:
1. A shocking ray--try to stay out of the red targeting circle;
2. A purple side-to-side ray--use one of the pillars for protection;
3. An electrified floor--jump to the edge of the room to avoid;
4. Two waves of robot attacks--take them out in the usual manner;
5. Guided missile attacks (2?)--Tough to avoid, use slow time and shoot them (?).
A good tactic to use is when the boss finishes the purple ray attack, she goes to the middle of the arena and uncovers her "face"--great time to slow time and blast away! Also, note that after two of the boss's four health bars are gone, a "friend" shows up and takes out the third bar.
It is likely that use of health packs will be necessary--probably best to pause the game to use them. After the boss is defeated, there is a short cut-scene and a decision to be made by the player (with no apparent consequences) .