Big Willy by physical standards is invulnerable thanks to a special alloy in his armor and is superior in strength. However his only weakness is his battery when he isn't fed on a regular basis his systems will shut down, thankfully he can be recharged by a weapon that generates electricity either the Zap-O-Matic or Ball Lightning would do the trick.
The Mech itself is equipped with 3 destructive weapons. The Regurgitron, Heat Ray, and Windbreaker are some of the most powerful weapons in the game, (the Windbreaker itself being second only to the Quantum Deconstructer). Use of any weapon will drain Big Willy's battery. You can recharge the battery by either, Leaving Big Willy and zapping it with the Zap-O-Matic or the Ball Lightning, eating the Brains of humans, or using a cheat in order to unlock Infinite Battery Power.
The first weapon equipped to Big Willy is the Regurgitron. This little device allows Big Willy to spew acid from his mouth in order to destroy whatever is in the acid's path. It barely drains the battery, but is extremely weak. Useful for missions that require that you take out a vehicle, but otherwise, steer clear from this weapon. It does not destroy buildings and does little damage to anything but a vehicle.
Heat Ray Edit
No Robot Mech would be complete without Heat Rays. The Heat Ray drains the battery greatly, only during prolonged use, and is the only Big Willy weapon to be usable while moving. Aim with the Wii-Mote and fire with the B-Button. This attack is good for taking out Bosses and Vehicles, but is not recommended for buildings.
The most powerful and most hilariously outrageous weapon of all which enables Willy to let it rip. The weapon spews out tons of corrosive material from Big Willy's behind. With the look, sound, and placement of the weapon, it actually looks as though the Mech is farting. This weapon can destroy buildings on contact, but drains the battery drastically. It is best to use this weapon after putting in the Infinite Battery Secret, which is unlocked using cheats.