You will have unlimited stamina -- keep running. Stay in the middle of the field until spikes or a bush pops up. Keep dodging the object and Nuget will run into them. Repeat the process until his health reaches 0.
Defeating RJ
You will first need to buy Verne's Spin Attack. RJ will walk around the cage slowly then he will start hitting golfballs at you. Hit the golfballs with the Spin and they will reflect back at RJ.
Defeating Vincent
When Vincent is about to attack, there should be a green circle. Step into the circle and he will miss. Then, start hitting his face. Keep repeating the process and he will fall off the truck.