1942: Joint Strike

Seems really challenging when you first play the game, but is actually really easy to do once you identify the Dead Zones for each of the bosses. The Dead Zones are basically areas for each of the bosses that you can hit their weakspot but they can't hit you. In essence you fly right to the Dead Zone and hammer away on the Fire Button. I'll discuss these Dead Zones in the next section but be warned, once you learn about them, much of the challenge associated with the bosses goes out the window. They die really quickly and more often than not you come out pretty much unscathed. So--avoid reading the Boss S Ratings Cutoff section (See "Padlocked" achievement description) if you are new to the game and don't want the boss strategy spoiled.

As far as Co-op versus Single Player for this achievement and "Padlocked", it seems like the bosses are harder to kill on Co-op.