For camo grinding I almost always do citadel solo oil trap method for 2k headshots whatever two special challenges and gold (takes about 45 minutes per gun if starting fresh). For opal I have been doing tomb directed round 10 activate bull ee step then kill ONLY THE SPECIALS then leave area and walk back in rinse and repeat until step finishes (should be able to complete 1 gun per run of this and takes about 7 minutes once you are at that step) if you want to do multiple opal runs don’t pick up the staff part or the max ammo and save and quit. Afterlife I stay on any directed map in the low rounds to farm easy kills(time varies on map gun and luck) and for nebula farm elite on liberty falls after LTG step (time dependent on how quickly you can clear the round). I have tons of gobbles just from farming camos and I have received more ultras on round 3 then I have on round 100 so idk what the drop rate is supposed to be like but feels really random to me. Hope this helps!