Monster Sanctuary

Duel Trainer Gotthelm can be found in the Duel Circle room. This is located to the left hand side of the Upper Sanctuary (pink area on map), and his room is marked with a diagonal double ended arrow.
You need to have the rank of at least 'Keeper Novice' to enter.

Gotthelm will Challenge you to 7 different duels:
Novice Challenge
Seeker Challenge
Lancer Challenge
Ranger Challenge
Knight Challenge
Champion Challenge
Dragoon Challenge

As far as I can tell these can be completed in any order.

Each duel will be a regular 6 vs 6 monster duel format. The novice challenge starts with 6 level 11-12 monsters, with each team getting harder to Dragoon Challenge which are at level 38. Depending on skill you will therefore have to wait until towards the end of the game to complete some of the later challenges.

Note the team composition changes for each of the 7 challenge levels, but won't change when re-matching at each level so feel free to customise your squad to counter theirs.

Successfully beat all 7 challenges to unlock the achievement. Should pop at the Victory screen of the final battle.