You will need 4 controllers to do this. The first thing you need to do is unlock all of the co-op attacks in the upgrade menu with each of the 4 rangers you'll be using. To do the co-op strike all players need to be in power ranger form. Fill the blue energy bar for one ranger by fighting enemies without using the blaster and finding blue orbs. With the energy bar full press LT with this player then press LT on the other controllers simultaneously to perform a co-op strike. You need to do a double, triple, and full team strike to get this. You do not need the strike to hit enemies or even have them around. Your best bet is to do this on the first level.