Unkillable Runic monster in Expedition (The real final boss)

Yesterday running my logbooks I had a runic monster spawn that was literally unkillable.
It was fire/phys/Chaos immune. I was doing Lightning dmg(Of course I was) but was unable to scratch it.
I understand more mods can made these more difficult but this wasn't just difficult it was god.

I offered a 1 Div reward for anyone in general to come in and kill this single monster.

Nobody could, the biggest juicers in the game came in, all the most OP endgame builds tried and died.

Most laughed and came in and said "ez" only to get wiped or leave the game.

Getting more difficult is one thing but this must be an oversight when a single mob can become actually unkillable with harder hitting one shots that Arbiter is a little out of line.

You likely chose the runic monsters regenerate xx every xx seconds remnant, and it also spawned regenerates life, ele res and is armoured.

Acrimony?