I ended up fighting Nim, with my purple BMG. The combat was moving too fast to switch weapons back and forth. So I just slugged it out with the old BMG.
Kept moving, attacking with the BMG and when I got hammered I use the secondary fire of the BMG to heal.
I also kept the power overcharge in use through the entire three phase fight. It allowed me to go from zero to 100% ammo on my BMG in an instant. So I would drain it first to attack Nim. Them hide behind something, pop overcharge to get the BMG back to 100% and I would drain it again healing.
Kept doing that and wore him down, finally defeating him.