Anytime I enter a battle with 20+ allowable Mana and Earth monsters are in the picture, it's almost automatic that your summoner will be Obsidian and your monsters will be those that use magic.
I would've chosen this same strategy too if I didn't have this one monster: Mushroom Seer.
Mushroom Seer does 2 magic damage and best of all... it reduces the damage done by magic attackers!
Like my opponent, I will still choose the Unicorn Mustang as my tank and the Goblic Psychic to heal it, but having the Mushroom Seer on my team helps me outlast my opponent.
This is a winning strategy 100% in this situation, and you can see for yourself here.