To kill him.

I suggest spectre armor with mana flower,tons of mana potions,rage potion,wrath potion,magic power potion, bowl of soup,sorcerer emblem spooky wings, mana weapon of choice preferably homing. Flatten some land to run across,also I suggest frosts park boots. Make lots of asphalt blocks put them in a extremely long line you will also need gravitation potion.

Once  ready, fish with truffle worm and summon him.flip gravity often as  the movement falling is faster than his movement speed.kill him till he gives bubble gun as the bubble gun can do serious damage with a good setup it can do real damage when setup right 

How to make shroomite/mushroom armor part 2

Okay,so you will need to go underground to find mushroom biome,destroy mushrooms with anything but pickaxe/drill and after destroying many mushrooms you shall get at least 1 mushroom seed(you may have to wait for mushrooms to regrow many times and destroy mushrooms many times before you get 1 mushroom grass seed) plant the seed in your 200-400 mud block above ground biome and afk it will take hours and hours for it to grow. Then once you have your own mushroom biome make sure all your other npcs have houses,I forgot once and had to make many houses in mushroom biome.

Once you have gotten the npc with a mushroom on his head,you need to get 1 platinum coin.to buys the auto hammer.

How to make shroomite/mushroom armor.part 1

This is

First you must be in hard mode and must have a pick axe or drill that has at least 200% mining power.

Next you mine chlorophyte(found in jungle) but remember to leave 1 chlorophyte ore as it grows back,but leave mud by the chlorophyte ore.next,for a full set of shroomite/mushroom armor you will need 324 chlorophyte ore which in a adamantite forge or titanium forge,you turn into chlorophyte bars.

You will get 54 chlorophyte bars from all the 324 ore. Next you will need a above ground mushroom biome, which you will need to place at least 200-400 mud blocks then place 1 mushroom grass seed in 1 block of the mud,

Piranha gun is epic!

This is the piranha gun.

The piranha gun is an item in Terraria that can be gotten in Terraria 1.2 and so is available to most Terraria players. To get it,you kill enemies in jungle biome post hard mode. You will have to kill al 3 mechanical bosses to get their souls and planters for the key then at a mithril anvil which you need 10 mithril bars or you can make orichalcum anvil which needs 10 orichalcum bars then you craft the piranha gun and hope you get unreal 😀 :)