1) Use all three rage skills: Earthquake, Call of the Ancients, Wrath of the Berserker.
**** All three skills simultaneously will destroy any mob group up to norm/NM/early Hell (thats how far i got as of now)****
2) Leave the game (10 second wait)
3) Quick-join your friend's game and click on his flag (~another 10 sec)
All your skills are now off cd.
rinse and repeat.
*does not guarentee boss fights* ^^
tl;dr follow steps to make 120 sec cds -> ~20sec cds