Basically on the quest Terror in the dread wastes - townlong steppes, you kill a 3million hp big beast dude worm thingy that offers 200k xp rested, and you simply delete the quest and re-take it or just sit his respawn while still on a completed quest or tag others. You are completely phased here making it possible to bot this if you could figure it out, im not sure if this is a guide or an exploit as im sure most quest mobs dont drop this much xp and allow for a re-kill - please watch the video to see me kill him in 30 secs on a level 89 mage. GL and leme know if this is worth it or not. If you got the dmg then i guess its worth it like me. XP will be more with banner ofc.
Enjoy!
Video : 0wnedcore townlong steps quest glitch - YouTube