Advice for the arbitrator?

[jhb14]

I’ve been playing for a few weeks. Finished ED1 13 times, ED2 3 times, ED3 2 times, and couldn’t get past the boss of ED4

My hero is at level 456/590 (I’m farming seeds overnight)

And I’ve unlocked and fully researched every familiar up to valley village

I unlocked the advanced spell books and the eagle/serpent/lion form

My equipment is a bit lacking but I got the magic wand up to +5 and the steel sword up to +3

Any advice on taking down the arbitrator? I can barely put a dent in him with my highest level spellcaster, scarlet dragon, silver scale dragon, and all my best “all stat boots”. Oh and I’m using the “beast king” title.

[Darnavir]

Mysterious body causes it to absorb magic damage - don't hit it with magic after it uses mysterious body. There's nothing else to it, more status up/gluttony/guardian's bane are needed if it doesn't die.