This is essentially poison's last hurrah; it's practically begging for you to use it with its massive HP pool. However, it's not quite that simple. It often uses Power Spike which cures poison and gives it a strength buff (and its STR is very high; I was fortunate to have Tio use her support craft when she did).
Sealing it will prevent it from curing and buffing itself. Instead it'll use Shadow Blade. This enemy also has port priority targeting with arts and is weak to time arts, so A-Reflex is another no-brainer to use. Most of your damage here will be from poison; it'll take forever otherwise even with the 250% earth weakness.
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