It's very difficult if you don't have good enough gear.
Figure out how it moves and try to avoid taking damage as much as possible, stock up on healing potions, and use ranged attacks. The boomerangs, bows, guns, and fast attack high damage swords work well... I believe flails work best.
Stay in an open area so you can hit it. He can move through blocks and most weapons can not. Try a vilethorn if you have to fight in a closed off space.
The real key is to take less damage than you can heal with potions, and do consistent damage.
If you don't have lesser healing potions, invest in them. If you have the better ones, all the better. If you have less than 400 hit points, consider hunting some more hearts.
Get (at least) 150 Shurikens, have 10 hearts, and a full stack of potions and you should be set. The Shurikens kill the minions in one hit. Just keep kiting it while healing when necessary and it'll be over in about eight minutes.
What tier of armor do you have right now (bronze, iron, silver gold, demonite etc...) Its very possible you just do not have high enough gear right now. As others have said, raise that max hp if you are not at 400 already.
yeah I went with shurikens. if you can get him to slow down above you, the shurikens will deal damage on the way up and back down. also, once he goes to his second phase i think bow/jesters works better since hes way easier to hit then.