To be honest, I found doing the Daily quests in game the best way to gain XP.
can start from "A Miner Problem" which is 300xp and takes 1 40 sec run to do, might not seem like alot but it all adds up
some quests can even offer upto 1000 xp so worth checking out those replayable quests ^.^
ps.. dont forget things like XP lixers, pets that give xp and even maybe making a xp awake gear set if you have the funds for awakes ^.^
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