The Wheel of Harma Trials become available after you finish the events at Angri-La in Act 2. To start the Trials, go to The Field of Discipline and speak to the High Lama. There are five overall trials you need to complete, and for the achievement you do not need to complete the battles within enough turns to achieve all rewards, although many rewards are outfits for your characters which you will need for another achievement.
To unlock the last of the trials you have to do wait until you go to Havens Above in Act 3/Post Game. I would recommend waiting until level 99 along with having the best gear you can before attempting the fifth trial.
Here is the strategy I used for each trial as shown by the FAQ guide on the game by Azmuch. I also had Catholicon Rings +3 for every character, though this was a little unneeded. Trial 1:
Again, going into this trial without being fully prepped and at level 99 will be a challenge. After every few turns or anytime you kill a monster, "back up" will show up, and if not done properly can send you into a infinite loop. I would recommend everyone being pepped before going into the fight with Veronica and Serena being mandatory in being pepped. Trial 5: