This method uses Gravekeeper's Vassal and Union Attack to increase Vassal's attack to more than 5,000 and deals effect damage for the extra bonus points. Having 2 Union Attack is always better than just using one to achieve "Over 9999 Damage". If you only managed to get 1 Union Attack, you can use Spell Reclamation to re-use it.
Millennium Scorpion has the ability to increase it's attack endlessly by 500 each time it destroys enemy monster. It can also be summoned easily by using Insect Imitation through any level 4 monster.
Piranha Army can be used as the finishing card to deal direct damage. Increase it's attack by using cards like Riryoku or any other attack increasing cards, it will have double the effect because of Piranha Army's effect.
This card can increase it's attack by 1,000 each time it destroys an enemy monster. It is much more trickier to use than Millennium Scorpion, since summoning this card requires an already face up Wicked-Breaking Flamberge - Baou on your side of the field that is equipped to a different monster, and Guardian Baou itself starts at a very low attack of 800.
Piercing Moray + Jam Breeding Machine
Piercing Moray has the ability to permanently increase his attack each turn by 700 attack. It requires a tribute of fish, sea serpent, or aqua-type monster, but with the help of Jam Breeding Machine, you won't have to keep summoning new monsters.
This card is available to purchase from the card trader. Victory Viper also has the ability to permanently increase its attack power indefinitely when it destroys an opponent's monster. You can also use the support spell Power Capsule to speed up the process.
Using a plant type deck, each plant monster in your graveyard will increase your plant monster's attack by 300. While it's lower than some of the attack increasing cards, if used with Talaya, Princess of Cherry Blossoms, you only need 8 plant monsters in the graveyard to reach over 5,000 attack. This can be used with Gravekeeper's Vassal and 2 Union Attack to reach over 10,000 damage.
"Titan Showdown" skill
Similar to Piranha Army's effect, Titan Showdown skill allows any monster card to be able to deal double damage. Using cards that already have high attack like Blue-Eyes Ultimate Dragon will make it easier to reach over 10,000 damage.
Fusion Deck for auto
The skill "Balance" is a drop skill that some characters can learn, although Mako can learn it by levelling up. This skill is used in the Two Fusion Dragons deck, which is a deck that's made to get a score of around 3,000 - 4,000 by using auto against Standard Duelists.
Most monsters LD uses are effect monsters. The angel will make most of their monsters ineffective since they can't use their abilities. Protect him with cards like Riryoku Field to make sure it doesn't get destroyed.
If dueling level 40 Legendary Duelist is difficult for you due to lack of cards, you can always farm the LDs at level 30 instead. An easy deck to make is to simply put monsters with 1600 attack or higher level 4 monsters that are glossy and/or prismatic and perform some tribute/fusion/ritual summons for some extra scores. Being able to spam level 30 LDs with a consistent high score is better compared to getting only 3,000 or 4,000 score against level 40.