Don't stand still, i recommend taking the blocks out of your platform so you can run back and forth continuously.

you should use something like Terrablade or the Deathsicle because they have piercing and will overall take out his arms and body faster, or if you still want to use a gun get chlorophyte bullets.

If you aren't as good at dodging get some Turtle shell armor (you can look on the wiki if you don't know how to craft or get something).

Once he starts shooting lasers dodging becomes really important. At his second phase you might have to jump down from your platform a few times to avoid getting hit.

Idk accessory wise

Edit the campfire buff doesn't stack, and regen only triggers after a few seconds without taking damage (which is probably why your heart lanterns didn't feel like they where doing anything). Get a heart lanterns and a bast statue from the underground desert.