I've just cycled this a few times but you can farm Dr lash MERCENARY ONLY unfortunately. Essentially get the railgun from chaurmine (you need all left perks for Merc well all that actually matter) take cover on first run use rapid fire 3 times (he'll hit you on the 3rd try) and take cover again then boom you win.
Time to farm before this gets patched
Time to farm before this gets patched