The final Spiritus goes out on a whimper, as it's super easy relative to our current units. The only real mechanic at play here seems to be the blue aura they get at the second and third HP thresholds. It seems to give them some HP Damage Reduction and makes their attacks very high turn rate. Dispelling it seems to only require taking two hits from them while the aura is up. So it encourages off-turn play. The enemies also can't be delayed even by breaking so that also encourages off-turn.
I didn't have to do anything specific here, I didn't even cast Beatrix's Safeguard as the bosses were already hitting for 0 HP Damage since they couldn't deal enough damage to get out of Break state with their BRV hits. This party also covers the Crystal and Series challenges for this stage.
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