I don't know what your spec is, but as a pyro, I consider myself a mobile melee/ranged hybrid. Many bosses have some sort of melee-range AoE, so I find myself at max range for Rail Shot/Rapid Shot/Unload, running in halfway for Flame Burst, running into melee range for Rocket Punch, and then running back out ASAP. If you're good at timing your abilities, you won't get hit by any boss AoE but you'll still do your maximum DPS.