Big Willy by physical standards is invulnerable thanks to a special alloy in his armour and has great strength. However his only weakness is his battery, when he isn't fed with Furon DNA from human brains 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.
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 Deconstructor). The use of any weapon will accelerate the draining of Big Willy's battery.
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.
The Heat Beam is the only weapon that can be used while moving. It is good for taking out vehicles, damaging bosses, and destroying buildings. Prolonged use of this weapon will drain the battery faster.
The most powerful and most hilariously outrageous weapon of all which enables Willy to let it rip. The weapon emits a shockwave 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.