Ok. I thought it would be simpler aura but as it turned out, I needed to do some coding.

Check this aura. I prepared for you three edit boxes so you can fill them with your name of sets. You have to go to triggers 1, 2, and three and use names of bosses. Obviously, ItemRack set 1 corresponds to Trigger 1 where you put the boss name in the filled "Name". Unfortunately, I can't test it.

It should work as follows:

When you target a boss and are not in combat, aura should change your gear set accordingly. When you leave the combat, it will return to your normal gear set. When you target anything else but bosses listed in Custom Options, nothing should happen.