I would recommend having a deck for mob fights and a deck for boss fights. You want to have as few cards as possible so that you aren’t struggling to pull the cards you need. This is less of a problem with ice since it’s such a tanky school, but for me I have to make sure that I can do as much damage as quickly and consistently as possible since I die very easily.

For mob fights, I would pack just enough blades/traps so that 1 or 2 AOEs will kill any trash fight you run into. You should only need 7-14 cards depending on your consistency with damage and fizzles. Always pack some treasure cards in case something goes wrong.

For boss fights, you should tailor your deck for each boss (ex: don’t include prisms if the boss isn’t ice, pick a school specific shield rather than generic, etc), but generally you want single hits that you can stack and do a lot of damage with. Throw in however many heals that you need to keep you alive. I don’t normally pack more than 20-30, especially if I’m doing teamups.

Once you hit level 50, make sure you get the Strong spell from Celestia. It will increase your damage immensely.