Ok, first the weapon:
You'll need a weapon called "last prism", it's dropped from the moon lord (the third strongest boss in Terraria: 1. blue slime, 2. dungeon guardian and 3. Moonlord)
It's really important, to have that item, because if you don't have it the blue slime has 999 defense, just like the DG.!
Now the armor:
You'll need the solar chestplate and the solar leggings..
As a helmet you'll need a Kingslime-mask!
The mask will protect you from the damage the slime deals, if you don't wear it, it does like 99.999 damage..
The next item you need is the royal gel. If you don't play expert mode, then you'll have to cheat it, you decide in wich way.
Anyways, it makes it impossible to the slime to attack you.

So:
Here's the guide, how to use your weapon:
1.:
Drag the last prism in the first hotbar slot and press the 1-key
2.:
Drag your cursor onto the strong enemy and hold leftclick.
3.:
If he survives it, you die lmao

The end