Hello, I'm currently Power Leveling 2 friends on Hardcore. Myself, 60 Wizard and another Friend, Barbarian.
Method:
(*Or what ever level of Torment the BOOSTERS can handle)
Level 60 starts a *Torment I Act 1 - Game on the VERY FIRST quest of the WHOLE GAME.
Invites lower level friends
Kill everything. Be safe. Watch the EXP FLY!
Enjoy guys.