but then I wouldn't be able to have that actor use any other elements, correct?
wrong, you misunderstood how elements are applied.
First of all, it's the skill that determines element.
There is no problem with one actor having several skills for different elements, like fireball and ice spike or whatever.
If you go this way then only some skills of that actor instead of all skills will damage that boss however.
then there is the setting "normal attack element". This setting in the skill means that the skill should look for the attack elements available to the actor.
That attack element can be set directly to the actor, although it is often added by weapon.
And contrary to your belief it is possible to have several different elements added this way, even if only one element is used in the attacks if that is the case.
I don't remember the function the engine uses to decide which element to use if several are available.
If I remember correctly it will be the one doing the most damage - which would be ideal for you.
But you can simply test that in your game.