@BlackPanther First I would look at the guardians class. If they were of the combat or weapon class, at best (meaning with character growth and all) I would say they would be able to perform SOME mid-level illusions. You know, trick the senses, maybe some of the illusions are capable of inflicting pain. On the other hand, if they were a caster, that's when you start getting into creating an entire zone of illusions and your enemy will most definitely feel the pain.
Secondly, if the guardian has another magic it will weaken its illusion magic. Personally since it's only two magics, I would cut it in half or balance out the proportions how you see fit. Maybe 75% of its power is in fire magic, with a mere 10% in illusion. In which case I would imagine the butterfly to be something like red wings (taking up the majority of the butterfly) with yellow tips. There's a lot of ways to mix it up, i'm trying to give you guys options :) As for how powerful it is.... Let's use, for example, the guardian is of the caster class and its power is split straight down the middle between both magics. Naturally, being a caster, they would still be able to cast higher level magics, but it won't be as "effective" (for lack of a better term) than a caster guardian that only uses illusions. The pure illusion caster may be able to create an illusion zone that spans 300 meters, while the other can only create a zone for 150.
At the end of the day, I'm giving you guys free reign over the abilities as long as it's within reason. So one guardian may not even cast the same illusions as another. Just remember, we won't be pulling out any city-crushing magic in the beginning of the roleplay lol. It's all about progression