Here's my approach. I consistently get 8000+ using Chaos Command Magician and the skill Three-Star Demotion.
Skill: Three-Star Demotion
Chaos Command Magician x3
Gravekeeper's Vassal x1
Union Attack x2
Supremacy Berry x3
Magical Mallet x3
Acid Rain x3
Secret Pass to the Treasures x1
* Initially I ran one Reload that I probably got during a wild encounter (roaming) in the Duel World. You can get one if you have an SR ticket.
** You don't need Storm. I didn't know what else to run. See my explanation below.
Comeback Victory 2500
Low LP 500
Cards on the Brink 500
Victory by Effect Damage Only 1000
Played Prismatic Card 200
Played Glossy Card 100
Over 9999 Damage! 3000
Destroyed 10 Monsters 300
Activated Spell 100
Performed Tribute Summon 300
Deck explanation below.
I use one prismatic and one glossy card. You want to summon Chaos Command Magician (CCM) on the first turn using the skill Three-Star Demotion. Use Reload or Magical Mallet to redraw for CCM on your first turn if possible. Once CCM is on the field, Bandit Keith will usually place his monsters in face-down defense position. Attack all his face-down monsters.
(cont'd) Do not let him tribute summon his stronger dragons. If he has a monster in face-up attack position, use Acid Rain to avoid dealing battle damage. Use the Supremacy Berry twice to raise your LP in order to summon a second CCM using the skill. Use the third berry to raise your LP, then use the skill again on another turn to lower your LP to 1000 without summoning anything.
(cont'd) At some point, tribute summon a CCM to get those duel assessment points. I do not recommend that you set any other cards or use other cards, like continuous spells, because this will encourage him to summon Twin-Barrel Dragon in attack position in order to use its effect to destroy your non-CCM cards.
(cont'd) This will give him a chance to tribute summon a stronger dragon on the next turn in case you don't have an Acid Rain ready and don't want to lose the "Victory by Effect Damage Only" points. Sometimes he still summons Twin-Barrel Dragon in attack position and tries its effect on your CCM! Anyway, draw cards until you have none left.
(cont'd) On your last turn, summon Gravekeeper's Vassal and use Secret Pass to the Treasures and both Union Attack cards on him. You will have one GK Vassal and two CCMs on the field. Then attack directly with GK Vassal for 10300 damage.
(cont'd)Finally, you don’t need the spell card Storm. I didn’t know what else to include. I used it to destroy his backrow once, but it was not necessary. Surprisingly, he had two Rare Metalmorphs set. He could have used that to destroy my monsters, but he didn’t. This leads me to believe that he waits for you to attack his monsters before he uses it.
I haven't tried it, but I assume he would try to summon Twin-Barrel Dragon in attack position to try to destroy the Mausoleum with the dragon's effect like I explained in my guide. If so, you will risk losing the "Effect Only" points and give him a chance to summon his stronger dragons. Also, he could use the Mausoleum for himself and summon a stronger dragon and destroy your CCM.