If you and your friend ever find yourself stuck, i'd suggest to try this method:

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