Once you obtain a second persona you can begin switching between the two.
Choosing 'Persona' in the battle menu and selecting a different persona than the one you currently have equipped will register as one switch.
However you can only switch personas once per turn. A quick and easy way to do this is to first ensure you set the battle tactics to give you full control over your party members. Switch personas every turn and guard/heal with party members until the trophy pops.