There are two buffs he has. One reflects ranged physical, the other magical.
Leviathan didn't have anything similar when you fought it?
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Target the tail.
it is the part of him that will not reflect Range Damage and is considered his Backside so you can perform position base attacks for maximum DPS.
When his tail appears, his head will reflect ranged physical attacks, so bards must target the tail. However the tail reflects magic, so yeah. The fight isn't an "OMG guys, can't use ranged", it's a "OMG guys, i actually have to pay attention and not just blindly attack whatever i want to".
So if we kill it's tail we get a leviathan drop, right?