I can't use White Death because there aren't always going to be reds and purples to proc Headhunter, and if I get into the trap of chunking away at an elites' health, I'll just as soon see yet _another_ Heartbreaker roll up and steal the kill. This isn't even on Legendary, I don't bother sniping on there unless it's on Floor 100.
I play MMR builds in Heroic all the time and I don't even use the White Death never mind the Nemesis, I just use the M700.
For solo (Screenshot below) I use Preservation, Perfect Bloodsucker and a second Belstone.
For group scaling I'll swap to Ranger, Perfect Vigilance and Contractors, I also switch the mask for a second Providence or W&H.
3 piece Aces and Eights with Airaldi can really push the damage numbers up as well and I roll Damage to Targets out of Cover on all my MMRs.
For Chungas I just use Headhunter, 2 headshots give me max damage so It's: pop 2 heads, hit the helmet if it doesn't crack, pop 2 more and try again.
If there are no reds/purples to proc Headhunter I use the Tac 50 which can usually one shot yellows if they're not named or wearing a helmet.
Mantis is OK with ChainKiller if you can put up with the horrible scope, the instant cool down on the decoy even makes it surprisingly good for solo Legendary.
I agree the slow pace with Nemesis is annoying, I gave up on it very quickly. I keep meaning to try it with the decoy to see if it makes a difference, I'd like to try it with Memento and maybe an additional skill tier for additional duration with Capacitor as secondary.
Would you like to share your build? It would be interesting to see what your running.