Titan Showdown is usable in some decks to farm Yugi Muto for both level 30 and 40. Since it can double the battle damage dealt, it makes reaching the over 9,999 damage much easier, especially if you don't have Union Attack or Piranha Army.
Level up Tea to at least level 3 to unlock her Holy Guard skill. This is mainly for farming using The Unhappy Girl against level 40 Yugi Muto. It isn't doable against level 30 since he has Labyrinth of Nightmare.
Since Cerberus deck can be used against level 40, try and get either Sorcery Conduit or Draw Sense: Light / Low Level. Cerberus decks still work quite well even if you don't have access to cards from the previous packs.
Any deck will work, abyssal designator comes fron SDs, which rekts gandoras and kuribohs, the last day of the witch comes from them as well, use sorcery conduit to get cerberus and start spamming spells that are easy to trigger, like gain life points or pay life points, just get rid of kuriboh and gandora