Are Gunners bossing classes?

Like everyone said, they lack party skills and they have low HP. Another problem is they also don't have "Soul Bullet" so they're going to run out of bullets rather quickly spamming Rapid Fire at larger bosses like Horntail and Pink Bean.

At least bullets have much higher quantities per slot, I believe it goes up past 2k per slot on the high level ones. Not sure how fast Rapid Fire consumes though, TT is about 10 stars per second.