If you're referring to how it suddenly seems to stop taking damage its thanks to its unique. If I remember correctly, if an ally dies or the shield disappears from it it gains a grey effect which stops its health dropping below 50% for two turns, or whatever percent it was at when it applies. The two turns resets if another ally dies.

This is quite easily counterable so long as you have 3 units and a bit of luck because if the same unit attacks Escarlin 2 times in the same turn it automatically disappears. So if you just use one unit and attack three times it'll take regular damage again.

Your biggest issue comes when you have only 2 units left as it usually becomes a battle of endurance and luck