Gray Zapinator is broken
Anyways, here's some tips for ya:
1. Build your arena long and tall. A arena with 4 platforms and a total of 80+ blocks tall as well as 120+ blocks long is perferred. You can use the arena for the mech bosses as well if needed.
2. Destroy the hands first! If you don't then the heads take no damage
3. for the second phase, circle the head to dodge the skulls. Grab a good hook and a shield for mobility (Don't forgot boot and a double jump)
4. when spinning in the first phase, avoid going up as that would likely get hit by the hand and knock you into the spinning head
5. When lacking mobility by getting hit by the hands and head is spinning, try bonk on skeletron and hook out, the tiny bit of knockback might just save you
6. The hands will not likely hit you if you just run in one direction. that being said, tho, sometimes the hand come swooping horizonally on your platfrom, go up or down to avoid it.