The first legendary enemy I fought was a Mirelurk, and it killed me. I reloaded. It killed me. I took lots of buffs, switched weapons, worked out a strategy, and tried again. It killed me.
Eventually, I back-tracked to Robotics Disposal Ground, and took the Fatman that I'd left behind because I don't usually use them. Went back. I killed it.
On that legendary Mirelurk was a pipe gun of wounding (i.e. +25 points of bleed damage). It ruled! The mirelurks resisted the actual bullets, but the wounding effect made short work of them. My first Deathclaw died to it. I kept upgrading the gun - as I got better gear, it slowly went from being my primary weapon to being secondary, to trash-sniper. I'm now level 115. I have lots of legendary weapons, but the one gun that hasn't left my inventory, and still sees regular use is that first legendary wounding pipe gun. The wounding effect doesn't really do anything any more, but that gun saw me through so many fights that I'm not putting that thing down.