Duke Fishron

 is a Hard Mode boss added in 1.2.4. In order to summon him, a player needs to fish using a Truffle Worm as bait at either of the oceans. It can be summoned directly after activating hardmode, but it's recommended to have at least post-Plantera gear equipped.

Attacks:

First stage: Second stage: (He's a lot darker and eyes glow yellow.) Third / Desperate stage: (Exclusive to Expert Mode, only activates at very low health)
 * Charges at a player. (5 times in a row.)
 * Detonating Bubble spray. (The bubbles are, like chaos balls, poppable.)
 * Summons two aqua rings, when the rings touch the ground, they transform into sharknados.
 * A sharknado is a huge tornado that throws a lot of Sharkrons at any direction. (Similar to the movie, Sharknado.)
 * Charges at a player again. (Three times in a row.)
 * Spins quickly and releases sharknados and homing bubbles.
 * Shoots one homing aqua ring. If this ring hits a player, then a larger sharknado (Named Cthulhunado.) will spawn beneath them, spawning more Sharkrons than a stage one sharknado.
 * Background will turn blue ocean, rendering Duke Fishron invisible.
 * Only his glowing eyes will be visible at all times, while his body will only be visible when upon charging at the player. He also teleports into random direction after each charges, making his attacks unpredictable.

Strategy
A very good defensive strategy that doesn't require many rare items would be to craft 2 Gravitation Potions, have around 70 defense, acquire the Terra Blade and some Warding accessories. Simply activate one potion and repeat moving from the top of the map to the sea level while swinging your Terra Blade. When Duke Fishron reaches stage 2, the player must try to avoid moving too far away from the boss as it will make it despawn. To do so, while the boss uses one of the spells that makes him stand still, gravity must be inverted. Using this method and some acceptable dodging skills, the player should be able to take out the boss in a maximum of 5 minutes. Without having to farm any Key Molds, it is a very general and effective way of taking down the boss. To defeat him with minimal damage, have some Greater Healing Potions prepared, decent defense (60 or above), and Vampire Knives. Vampire knives and 60 defense aren't needed if the player has high DPS magic weapons. With the Spectre Hood Set, it's best to use high DPS magic weapons instead of Vampire Knives. It is also best to get a Hoverboard, or build a skybridge with Asphalt Blocks. Do not start using the healing potions before it is enraged. While the boss is charging, shoot towards him with Vampire Knives, while moving away from him with a Hoverboard or skybridge. This works quite well as the Hoverboard is almost as fast as the boss when charging. It is important to keep moving away while shooting when the boss is shooting bubbles or using the sharknado attack, as it spares more time for the player to heal. When enraged, the player might find it a bit harder to avoid its attacks, thus allowing the greater healing potions to come in handy.

Another strategy that is very simple, is to acquire the Piranha Gun and simply kite him around on an asphalt surface, preferably around 2000 blocks long. With decent dodging, high defense and a couple of health potions in reserve, the fight is easy.

Once the Flairon is obtained, the battle can be over in seconds. Just keep running away or avoiding the boss while shooting the Flairon towards the boss, and the battle should be won in under 30 seconds without the need to heal.