When Dreadnought begins leaking oil from the tank at the back, get behind it and use Ga-boi. The back will turn red to show that its shield is damaged. Continue attacking the tank to remove the shield from the legs. Now focus on the legs with Gargantua. Once all four legs are stripped of armour, attack them further with Degasser to knock the Dreadnought down. Quickly run over to the head and attack it with Degasser until it starts to get up. You will know when he is going to get up because he will shoot fire out of his mouth. When this happens, start attacking his legs with Gargantua until he falls again. Keep repeating this process until its Id turns red.
Tip - Do the beat absorption as fast as possible. If done too slowly, the absorption will fail and the fight will continue.
Use Volcano to attack the tank at the back of Dreadnought. Keep attacking until it collapses to remove its shields. Defend with Patriot and then attack the legs using Brummbear to blow the armour off. Use Hawk on the unarmoured legs to knock down the Dreadnought. Quickly attack the head with Ga-dearg until fire comes out of its mouth. When it starts to get back up attack his legs again to repeat until Dreadnought is defeated.