You get a Duel Assessment of 6,000 by performing tribute/ritual/fusion summon, destroy 10 monsters Yugi Muto controls, activating Spell/Trap, and finish a duel with over 5,000 damage when your deck is 0.
Using the Cu Chulainn
Ritual summon Cu Chulainn when you have 0 cards left in your deck. Use Secret Pass to the Treasures on him and activate his ability by using Blue-Eyes, that's in the graveyard, to boost his attack by 3,000. Equip him with a ritual weapon and use Riryoku to further increase his attack to deal over 5,000 damage. If your Blue Eyes is not in the graveyard, you can tribute summon him to the field first, then use it as a sacrifice for the ritual summon.
Make sure that Yugi already used all 3 kuribohs before you attack with Cu
This is mostly used to force Yugi to throw out his Gandora. This will reduce the cards in his deck so you will need Jar of Greed to match the number. Abyssal can also be used to look at your opponent's hand, by targeting a card that they don't have. Can be good if you want to check if Yugi has a Kuriboh in hand or not.
Get Yugi Muto to discard Sphere Kuriboh
To successfully finish the duel with Cu's over 5000 damage, you have to deal with Yugi Muto's 3 copies of Sphere Kuriboh. Either bait him into using his Kuriboh by attacking his monsters, or directly when you are sure he has Kuriboh(s) in his hand.
You can also use Abyssal Designator to force him to throw out his Kuribohs. To make sure it doesn't reduce the number of cards in his deck, use it when he's low on cards, preferably when you are about to attack on your last turn. Since 1 Abyssal is used to throw out Gandora, you will need to at least force him into using 1 of his Kuriboh so you can use the other abyssals for the other 2.
Boosting your score
Put any glossy/prismatic cards that you have to further boost your score. Each glossy cards increases your score by 100, while prismatic gives 200 each. You can replace the Cu Chulainn and the support cards to add in some prismatic cards to get a higher score.
Been using this deck to farm muto lvl 30 and quite well, the assessment sometimes over 9000
This deck is good but have some issues. Twice my start hand was something like Chulain, Riryoku, Ritual Weapon, Secret Pass and Abyssal If Yugi atak, in your turn he can use the trap to change your next monster to defense position, kill him and next atak direct.
1x Cu Culainn
1x Piranha Amry
1x Bewd (prisma)
3x 2H King Rex
1x Abyssal Designator
1x Emblem of the Awakening
1x Secret Pass to the Treasures
1x Jar of Greed
attack facedowns with your monsters to get out his 3x Kuribo, be carefull with union attack, it will kill your Blue eys.
have fun with 7k score
take out 1 dino and 1 abyssal.
take in 1 secret passage and 1 piranha.