Well, you'll either need to collect the cards from Arena booster packs for the sets they came in, or else directly 'craft' the specific cards you need using MTG Arena Wildcards - effectively tokens you can exchange for the exact Common, Uncommon, Rare, or Mythic Rare card you want, depending on the type of Wildcard
In addition to first-time battle rewards and chance rewards, each battle comes with a selection of tasks, such as knocking out cards with Pokmon of specific types, using cards of specific rarities, and winning within a certain amount of turns
Mewtwo-EX, on the other hand, could use X Ball with a single Double Colorless Energy and use the opponents high Energy costs against them
equip 1 Dragon or Machine monster from either GY to it as an Equip Spell that gives it 1000 ATK
I'd start out with one or two tutors that can get your wincons, but if you start working your way up the power scale, having more will be necessary for keeping up with your pod
If Scoop Up was around, you could then move Iron Valiant Ex without skipping an attack after firing off a Lazer Blade