I think Merc is correct except maybe if you try Approach 2B:
Approach 2: Open World tagging of mobs by character(s) to be boosted and then killing by main(s) has 2 different sets of levels where it is viable:
-2A) ...
-2B) Killing by level 60 Pet(s) of any level mobs can help after the characters to be boosted tag the mobs. Note that there does need to be 2+ alts in the party to be boosted for the XP to scale well.
If you have a pet class main and can try it please report back with your results...
If you have a Warlock you could use an 'eye of killrog' to bring huge packs of level 10s (or what ever low level mobs) to you and your voidwalker and get the void to start taunting & tanking... Your low level alt should then be able to AoE Tag all the mobs with consecration, AE or something... Then cast Thorns/fireshield and/or Retribution aura on the Voidwalker and let the mobs quickly beat themselves to death and otherwise have the void attack any that try to flee... That or maybe there is a Hunter's pet with a decent AoE damage affect that can wipe masses of lower level mobs.
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