Big Willy, by physical standards, was invulnerable thanks to a special alloy in his armor and was superior in strength. However Big Willy's only weakness was his battery: when he wasn't fed on a regular basis, his systems shut down. Thankfully, Big Willy was recharged by a weapon that generated electricity: the Zap-O-Matic or Ball Lightning did the trick.
Big Willy was equipped with 3 destructive Weapons: The Regurgitron, Heat Ray, and Windbreaker were some of the most powerful weapons in the game, (the Windbreaker itself was second only to the Quantum Deconstructor). Use of any weapon drained Big Willy's battery. You recharged the battery by: Leaving Big Willy and zapping it with the Zap-O-Matic or the Ball Lightning, eating the Brains of Humans, or using a cheat code in order to unlock Infinite Battery Power.
The first weapon equipped to Big Willy was the Regurgitron. That little device allowed Big Willy to spew acid from his mouth in order to destroy whatever was in the acid's path. It barely drained the battery, but was extremely weak. It was useful for missions that required a vehicle, but otherwise, steer clear from this weapon. It did not destroy buildings and did little damage to anything but a vehicle.
Heat Ray Edit
No Robot Mech would be complete without Heat Rays. The Heat Ray drained the battery greatly, only during prolonged use, and was the only Big Willy Weapon that was usable while he moved. Aim with the Wii-Mote and fire with the B-Button. That attack was good for taking out Bosses and Vehicles, but was not recommended for buildings.
The most powerful and most hilariously outrageous weapon of all which enabled Big Willy to let it rip: The weapon spewed out tons of corrosive material from Big Willy's behind. With the look, sound, and placement of the weapon, it actually looked as though Big Willy farted. That weapon destroyed buildings on contact, but drained the battery drastically. It was best to use that weapon after you put in the Infinite Battery Secret, which was unlocked by use of a cheat code.