I think the major reason why you don't see any is probably because the lag on Beast Tamer is insane. You lag when you hit mobs over 3 monsters. I doubt they have it any better in boss runs. A lot of their skills end up glitched.
That being said, I don't think they're particularly good bossers.
Wellll... i personally love my Beast Tamer. The bear is more of a bossing animal because it hits hard and fast. As for the Leopard, not at all. haha. I started bossing when i was level 130 on my Bear. The bear is sort-of-but-not-really tanky.
The main problem is Bear simply doesn't have enough mobility for later end game bosses such as Hellux and Magnus. By the time you get Fort to stop hammering, you've already been murdered ruthlessly. Even if you do manage to dodge, it takes a grip to get back into furious swipes which really cuts back on DPS...Maybe if one was able to get attk speed as an inner ability coupled with the Green MPE potions...?
well bt cant really cant kill magnus normal or hard on its own at all
it can do hilla, and czak the basic bosses and all but that really about it
//unless its super funded i guess, mines pretty funded but not crazy-like