Grenade  Launcher is the best because you can chain stun  them  so they don't regen or summon more an  the nurglins get slaughtered by is  so far  is   a  spectacle   =)

Standard Bearers have ability to place a Standard of Nurgle. It provides constant healing to all enemies within it's range, maybe buffs their damage as well. It's targetable so you should destroy any standard you see first. As others noted you can use knock down/stun weapons to prevent Standard Bearer from placing them. When fighting those enemies you need either a weapon with AoE effect (Heavy Flamer, Grenade Launcher or Frag burst mode of bolters) to damage Bearer, Standard and Nurglings at same time, or very high DPS weapon and switch targets (destroy standard as it appears - you will know he is about to place one by animation) or knockdown type weapon to stunlock him to death. 

inferno pistol + plasma pistol. Kills anything super quick, just watch your overheat bars.....

no his banners all do that he places.  they  just get buffed by them    shown by a red aura.  I recommend heavy boltor #2 ability  or  grenade  launcher  if you want to be at range.  As both of these have knock down that will slow or  stop their regeneration and  area  effect to kill the nurgling rushes.

If you use a large melee weapon with an AOE effect i used a thunder hammer or you can use a shield and power hammer the #2 effect for that combination is an AOE also. Also it helps a lot if your weapon has an interrupt of some sort to counter the healing of the standard bearers.

 That is enough to deal with the nurglings, then kite everything out of the room and kill it, make sure you have some health regen otherwise kiting will just wear down your inoculators to zero. Try if you can to get an item that increases your move speed it helps kiting. You cannot outrun the nurglings but if you have an AOE attack that regens every 5-7 seconds or so you can kill them very easily. I hope that helps.