Disgaea 3: Absence of Detention

This one isn't too bad. Basically, you need to kill an enemy with a combo of at least 11. Theoretically, you could stack 10 of your pals on top of each other and unleash the combo, but even if everyone were to land a hit, you’re only looking at a combo of 10.

There's a char that you can create called "Baciel" that has a base Evilty that allows em to attack twice as long as they haven't moved. Create 6 of these (just to be sure, you will be making them attack a stronger enemy and attacks can fail), deploy them and a stronger character in battle and make them all attack a single enemy. They should kill the enemy with 13 hits in a perfect scenario.