Use Golden Ladybug and Solemn Wishes to heal every turn.
Have a Cloudian monster and The Sanctuary in the Sky on the field to stall the duel.
You can search out The Sanctuary in the Sky with Zeradias, Herald of Heaven.
Use Divine Punishment to negate problem cards like Dark Magic Attack. You can also negate his search cards like Blue Dragon Summoner, Legion the Fiend Jester, and Sage's Stone so he doesn't deck out too quickly.
You should have 3 Cloudian monsters on the field before your final turn as tribute for The Winged Dragon of Ra.
Elements Unite! You will start the duel with 500 Life Points and your starting hand will be 1 card, which is "Gate Guardian". In addition, you will start the Duel with Suijin, Kazejin, and Sanga of the Thunder on the side of your field. Their effects are negated and they cannot attack as long as they are face-up on the field.
Honnestly, I've been killing him at least 30 times with 8000+ score (at least threee times more with less than 8000 score). An I still don't have all the cards. I have half of it only ! The drop rate in incredibly low, I never saw that :( and that's a pity because the challenge is high so we deserve good reward ! :(
I am using my Blue-Eyes Cosmo brain deck against him to farm him with quick victories 3-4k points, it is the fastest, most efficient way imo, as his deck is outright broken. It's not favorable for us to prolong the duel against him.
I've successfully used an Elements Unite farm twice; he just hasn't done anything during it. Do note that, in theory, he can get a monster up to decently high attack values, so summoning the second monster to power up your Gravekeeper's Vassal before the last turn can be risky, but I he usually won't keep them in attack position, at least from what I've seen.
Looking at the list here (assuming you're not mistakenly talking about LV40, and that this list is accurate), it's not possible for him to get a monster stronger than Gate Guardian. Even Dark Magician Girl with Formula and 2 DM in Grave is only 3300.
I'm going to try it out again with a Dark Paladin deck, but this time I'll add a Gravedigger Goul to banish some Dark Magician names so Dark Magician Girl doesn't get stronger than Dark Paladin. Another method is to use Ishizu's skill "No Mortal Can Resist" to change all of Yugi's graveyard monsters into Skull Servants.