Well, there's the ones I listed:
Bosses: Comrade's Hammer can one-shot anything from what I've seen, provided you have the handgun and Tech perks. If by some chance it doesn't one-shot, you can use a legendary DR12 Quasar instead and there is definitely nothing that can survive a full magazine from it. It crits constantly, which triggers Short Circuit over and over, ensuring they can't do anything back either.
Trash: Divided We Stand is a smart AR that targets up to 5 things at a time. You just point it in the general direction of a camp of enemies and they all die, nearly always without even needing to reload.
My iconics aren't even legendary yet, since I wait until L50 to do the final upgrade (because AFAIK you cannot re-craft them, only upgrade, and that gets ungodly expensive when you try to do more than a couple levels).
Yeah, that's the biggest reason I wouldn't use mantis for a full melee build after messing around with it on my other builds prior to saving up the E$ for gorilla arms. You can kill a whole group of enemies with a katana in the time it takes the mantis animation to play, and if you were using a katana you'd be in slo-mo (from Kerenzikov or Synaptic Accelerator) the whole time. With mantis you have to kill the rest of the group with them moving at full speed.