You need to keep damaging it over and over again, it has an extremely high health bar.

The Beast Claw's weapon is good for evading it.

Hey there!
My tip would be: High Attack Speed and Critical Chance, and if you manage to find the "+100% knockback" Relic it will come in handy as well so you can keep damaging it consistently without needing to move ;)

I have killed it many times. The key really is just knockback. If you can knock it back you can stand around and kill it -- it takes a ton of damage, more than any boss by far. The frog summons are quite good at knocking it back if you have them, as well as the repel wave secondary attack or the attack-speed shout secondary attack. Spam the thing with some knockback and you will find it (relatively) easy. Note, you can also kill it if you find a large vertical area and have good enough movement speed and skill to dodge around it constantly ... but that takes forever.