Veemon deck is by far the strongest to use. Look out for really good drops (red cards, ToyAgumon, etc.) Learn how to fuse cards that may be useful for the "A" battle, such as ToyAgumon, Psychemon etc. Usually past Flame City, enemies will always go for a lethal attack (whichever move will knock your HP to 0, usually the O move), there are exceptions sometimes though. The AI is weird. Beginner City and Flame City seems random. Make sure to get the Digiegg of Courage which allows you to use Flamedramon, and make sure to equip your partner Veemon with strong abilities before Igloo city (2x damage to water is really helpful here) To add on to the prior point, make sure to add all D-Otamamon and Meramon to your deck when going through Igloo City. It's by far the hardest part of the game so don't be discouraged if you lose multiple runs there.
Cant think of too much more than that. You'll see top runners use Japanese because the text is slightly faster, but English isn't that much slower if you're more comfortable with that. Good luck!
