All of his Pokemon are Fighting, so Psychics will work wonders, especially the Ghost line
If you have those out and then cast the new card Gatta and Luzza, you can immediately win the game
In fact, you should be playing some form of nonbasic land removal in your Commander decks, especially when it only costs you a basic land slot (Im thinking Field of Ruin and its cousins)
There were eight key creature types to focus on (Giants and seven other ones I can't recall at the moment), with changeling being a versatile keyword that bridged those creature types
Creature - Human Noble Color tag templates(None) Isolation Cell Whenever an opponent casts a creature spell, that player loses 2 life unless they pay
You'll be making the most of the Goad mechanic, which makes players attack each turn if able, and, if possible, any player other than you