If you are talking about the water guardian found in water temples, there is no special way to kill it besides hitting it with a sword, bow, cactus, lava, or harming potions.
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The Guardian
The Elder Guardian in the ocean monument is the third boss.
Although the main goal in Minecraft is to go to the End dimension and defeat the Enderdragon, it may be harder to create the Wither and defeat it. Once you defeat it, it will drop a Nether Star, which can be crafted into a beacon. If you make a pyramid of iron, gold, diamond, or emerald (or some of each), the beacon will give you special buffs. This is probably the hardest thing to do in Minecraft.
lol your a noob
3 {or 4 if you believe in herobrine} the 3 are Ender Dragon, Wither and the Elder Guardian