Depends on what boss you are going for. Swords and Grenade launchers are great when the modifier is Heavy weight. But other than that, the best "legendary heavy" is probably the lvl 2 reset for Gambit. Gives you a godroll Bad Omens with impact casing, tracking and cluster. As for Boss spec mods, just check back with Banshee everyday until he sells a boss spec mod. I bought 10 of each of his mods, but I never use them.

sins of the past. any rocket with cluster bombs. and holstered reloader is always nice for power weapons. i think my boss spec mods came form me buying them from banshee back when i actually had mod components haha

Zenobia-D rocket launcher with tracking and cluster bombs. But honestly - if you're not using Sleeper or the Ikelos Shotgun, you are not doing enough boss DPS.

Play of the Game, rockets.

Edge Transit