Use the lower level monsters and keep destroying keith's monsters. Don't attack if he has a face-down trap since it could be rare-metalmoprh. Set a magnetic mosquito if you think he will tribute summon either blowback or barrel dragon. Use acid raid if you don't have mosquito to destroy his tribute monsters. It's best to summon BEUD during second to last turn to avoid it getting destroyed early. On the last turn, summon vassal and use the union attack combo.
Summon either the hunter or jerry and destroy keith's monsters when they are in defense position to not lower his LP. If they are summoned in attack position, switch it to defense using otohime on the next turn. Acid raid if their attack is higher than yours, or mirror wall to defend as well as reducing your LP. Lower your LP to more than half of keith's LP and keep destroying his monsters so the BEUD can get a direct attack to deal 10,000 damage after equipped with sword of deep-seated.
Since the ATK of Twin-Barrel Dragon is 1700 and same to that of Axe Raider, the highest ATK level 4 monster you can get, you need to use equips spells or other ATK boost cards to get your monsters to defeat the dragon.
Use Spell/Trap removals
Destroying Keith's Machine Conversion Factory, Second Coin Toss, and Rare Metalmorph is important to help your monsters to beat Keith's Machine monsters.
Heya! I've made a pretty consistent 7.3k Labyrinth Builder deck for this guy. Hope it helps you all farm him.
x1 Gravekeeper's Vassal
x2 Cat of Ill Omen
x3 Cosmic Compass
x2 Union Attack
x3 Stray Lambs
x1 Shield and Sword
x1 Secret Pass to the Treasures
x3 Good Goblin Housekeeping
x3 Tyrant's Tirade
x1 Blue-Eyes Ultimate Dragon
How to use: Stall turns with Stray Lambs/Cat/Compass untill you have x2 cards in hand for Labyrinth, x1 Tyrant's Tirade, and either x1 Stray Lambs or x1 Cosmic Compass. Get your tribute fodder out with your Wall, activate your trap, pass turns and draw cards with Good Goblin Housekeeping, last turn activate Shield and Sword, set 1 extra card and special summon Anteatereatingant, change Wall to attack mode, summon Vassal, secret pass, union, union attack for 10k.