The 3rd challenge of the Trails of Cold Steel crossover Secret Realm event is the first one that lets you use your own party members, so if you've got a barracks full of weaklings... buckle in!
The primary gimmick of this fight is the cannons around the edge of the field. Once they've been activated by the boss, they'll shoot in 3 lines all the way across the map, tagging you with a debuff that makes you take way more damage. So you're going to want to figure out where your safe box is and use your free post-action move again to stay within it.
Other than that, try not to get too swamped by his AoE & fixed damage debuff, try to stay out of the way of his chain hook attack (or bait it with a summon), and try to erase the golems at the end of the turn rotation before they can mess with your positioning and you should be fine.
Let me know in the comments what kind of team you used! For comparison, here are the power levels of the team I used to win:
30