The 2nd challenge of the Trails of Cold Steel crossover Secret Realm event is another one that uses a set team. It's trickier than the first one, but you can still beat it without having to worry about your own barracks full of weaklings.
The two keys to this battle are staying spread out so Rean can't keep cooling down his skills, and blowing up your recruited golems at key moments. Ideally, you want to hang onto the silence golem until later in the fight, so you can prevent Rean from using his dangerous AoEs when he enters ogre mode at 40% health. If you combo the golems like I do here, you can get a couple turns of uninterrupted DPS.
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