I would try to stay away from melee when fighting bosses like Fishron. I would try to using guns like the megashark, but if you want to use melee the Terrablade, influx waver, seedler, and if you have killed the empress of light starlight is really good for pelting him during is phase transitions.
If anyone need helps killing a boss i guarantee i can kill any one of them, yes i can kill the moonlord in 20 secs or less, i can also kill empress of light. Everything the boss drops is completely yours. so just reply if you need anything or if you want to complain.