Kaiba possesses a few good cards like BEWD, Milenium shield and labyrinth wall. You'll mainly want to get a twin headed thunder dragon with a few boost but watch out for raigeki cause it will smash all of your attacking cards and also harpies feather duster I had a raigeki ready to use then he used a harpie he has little traps but you cannot win BEWD from him. He is also the grand finalist in the tornament when you play as yugi not the prince
after doing this you will return as your prince and go to the neteopolis and the dueling ground it's not hard and beat the Mage at the ruins in the metropolis you cannot go to heishin a castle yet. After that go to the kings valley. Seto will appear and tell you about high mages then check out map and drawing then after that leave and go to a sea shrine for a two headed boost or mountain mountain is a little easier but with sea they use all Neptune like Isis don't use any mars types.
When you STEC kaiba he gives you rare cards also high meadow Mage so get more rare cards from Jono2 like red eyes skull and meteor b dragon
ill be with more when I progress through a bit