Is there a fast way to unlock Torment mode?

How do you unlock Torment? DO I just need to beat Malthael? So if I join someones game before the last boss can I just fight him and unlock it.

Solution 1:

You only need one of your characters reach Level 60 to unlock Torment difficulty.

Here's the game guide.

Solution 2:

How do you unlock Torment?

If you reach level 60 with one of your character you unlock Torment difficulty. See here: Game Guide - Diablo III

You’ll unlock Master difficulty when any of your characters completes the game, and Torment difficulty when any of your characters reaches level 60.

Is there a fast way to unlock Torment mode?

Yeah, fast leveling! Ask a friend or ingame, if he can help you leveling. Make a game on highest difficulty and let your friend kill everything. If you killed Malthael you unlock Adventure Mode. Do Bounties on highest difficulty and earn even more experience.