Big Bertha is obtained by defeating a variety of bosses found not only in the Overworld but also in other dimensions. It is crafted using 3 separately constructed items: The handle, the guard, and the blade. It does 500 attack damage, has 9000 durability and has an extended reach. The way this is achieved is when you swing the sword, it will fire two invisible projectiles that do a fixed damage. Therefore a strength potion will not work at long distances, whereas within close proximity the regular sword hit will be amplified by the strength potion.
Here is a list of Bosses required for the 3 parts:
For the handle: Water Dragon, Mantis, Hercules Beetle, Triffid and Robo Warrior.
For the guard: Emperor Scorpion, Basilisk, Nightmare, Jumpy Bug, Mothra, Molenoid and Sea Monster.
For the blade: Sea Viper, CaterKiller, Vortex, T Rex, Large Worm and The Kraken.
Combine these 3 parts in a diagonal formation from bottom left to top right in your crafting grid to obtain Big Bertha. Combining Big Bertha and an iron ingot in a crafting table will allow you to create Slice (a white version of Big Bertha).