So with the new season theres a new legendary gem called the gem of ease which sockets into a weapon the gem gives a set amount of xp per monster death with a gem leveled to 25 or higher it makes any weapon level requirement 1.
The part that isn't intended is the fact that the person who is being power leveled can just sit in town and they get the gem xp from every monster killed by anyone else in the game
obviously the XP would be higher if you were in the area.
also because the XP its tied to the gem it doesnt matter what difficulty you play in either you could speed run areas in the lowest difficulty you just need mass amounts of monster kills.
you could level 3 bots at once using this method in very little time.
i power leveled a friend and it took 8 mins from level 1 to 70 but that was running t6 rifts but i think it would be faster just killing mass amounts of monsters in a lower difficulty.