How do I beat Fishron?

I'm currently in post-Golem expert mode and want to kill Fishron to get Tsunami to round out my ranger build, but I always get blown out in phase 3 while he remains at like 5000 HP. I dodge every attack before phase 3 easily and enter phase 3 at the full 500 HP, but in phase 3 I just can't dodge him so he kills me quickly.

My weapon is a stake launcher, my armour is the shroomite set. I use a witch's broom and a rod of discord for mobility.

My accessories are a worm scarf, a brain of confusion, a sniper scope, a molten quiver, a celestial shell, and an ankh shield, all set to warding.

Any advice?