The One in a Million badge, as you probably know, is for doing one million damage in a single hit. Therefore, your goal is to get the highest damage spell you have and buff it as much as possible. You may be able to get a treasure card for a higher damage spell than you currently know, so the Bazaar is definitely worth checking into. You also want to find as many stackable blades and traps as you can (stackable simply means that they will all apply to the same spell at once). A general rule is to look for cards that do different amounts of buff, as these will always stack (a stormblade adding 30% vs 40% and so on).
Based on your school and level, you should be able to find: balanceblade, dragonblade, elemental blades (both treasure card and regular if trained), treasure card school blades, any pets with a blade (maycast or card), gear with blades (amulets are also easily found in the bazaar), regular traps, elemental traps, and more. If you manage to find a sharpen treasure card, using that on a regular blade will also make it stack with itself. As a storm, you should also use your school star aura, your arena-wide storm spell (is it called darkwind? I forget), windstorm, and definitely spend the time to get as many pips as possible and use supercharge.
For the actual cast, the best method is to find a friend and do a practice pvp or go to a home dual circle so you can be uninterrupted and not worry about being hit. If this is impossible, then go for a weak enemy that won't damage you much while you stack. If you go the enemy route, you can take the time to find someone with weakness to storm as a bonus. Just make sure you are hidden enough that someone won't accidentally join your battle! Good Luck!
