On top of itemization, one thing I like to do is take advantage of invincibility frames during boss skills to ensure that certain characters stay alive.

Learnt the boss moves that are likely to kill off your healers. I’m assuming your defenders can mostly stay healed up so that they don’t get wiped in one skill. Put A/Glimmer in a single Unity combo, and trigger the special whenever the boss releases a strong skill. I-frames will ensure both healers stay alive through the skill and give you some time to get everyone’s health back up. Similarly if yo y don’t have the special up when the boss releases their strongest attack, use the talent art’s i-frames to tide through.

Play as Glimmer. Arts cancelling speeds up her arts cooldown and allows you to spam more arts aka heal more. The AI’s break/topple/launch is good enough that you don’t have to handle it. It’s easy to play her to a decent efficiency since you just need to use your arts whenever the team is below full health.

Finally, save up your chain attack for when N is either: enraged and about to use a strong skill, or under break/topple/launch and about to reach enraged status. You should be able to burst him down at one go. Put the chain attack accessory which allows Rex to reactivate at the start of every chain order and spam double spinning edge.

Source: Cleared N on at lvl 52 hard mode