Use King of the Swamp depending on what you need. You can send it to the Graveyard to get Polymerization or use it as a Fusion Substitue. It's recommended to have more copies of this card and replace other Fusion Substitute monsters.
Don't destroy his monsters so he won't deck out faster than you.
You also need 1 fusion twin headed thunder dragon. 1st turn summon Gate guardian on defense Mode. Draw all your deck and on last turn summon Vassal,activate Treasure pass and both unions,change Gate guardian to atk and fusion thunder dragon.Go to Battle step and activate antimagic arrows and direct attack.Ez 8k