You may choose only a creature or planeswalker thats already on the battlefield.
When you are up against a combo deck, you basically have one of two choices: Either you race, killing the combo player faster, than he kills you (and this often means FAST)
I would still prefer building a Metal type deck, using Solgaleo Prism Star and Dusk Mane Necrozma GX, just because Dusk Mane Necrozma is guaranteed to do 220 damage per turn, and Gardevoir GX is weak to Metal type Pokemon, but if you have any high attack cost Psychic Pokemon, Lunala Prism Star is a must
Pull this in your Build & Battle box and this could cause some havoc
Spinning Wheel is a handy tool that costs three mana and can be tapped for any color of mana, while also having the ability to tap a creature to prevent it from attacking or blocking