The main weapon of this 'bot is called the War Scythe.
Drive: 1 L-Motor
Steering: 1 Servo Motor
Moving Bucket: 1 XL-Motor
War Scythe: 2 L-Motors
The bucket moves up and down to fit inside of the 45x45x45 max size. It has steering and driving like a normal car to keep it simple and less likely to break. The bucket traps the opponent and pins it to the wall, while the scythe smashes it to bits while it is imprisoned.
Don't forget the SBrick!
Have fun and happy building!
Also, I know LDD is hard to use for instructions, so I divided it into smaller sub-asseblies to make it easier.