So, sort of like the Thousand Arms battle system? (google it if you dont know)
I would think this could be done without a script, unless you wanted it to look different graphically. otherwise, you could make an event that changes your players character in battle, and make a skill that calls that event. ( i THINK. ill have to experiment with it.) Do you need the monsters to be able to use this system as well? Cuz that might be trickier, depending on what youve got in mind..