It was one of the occasional 99 mana level battle. I thought that requires some high power. My Diamond Dragon won't suffice thought its magic is helpful. Thanks for some help from the summoner of @koskl. I didn't pick the cards from the highest level first but put in Sea Monster which I also like. I know it doesn't come with shield but a portion of the health restore each round which is good for business.
A few more support from others including a rather new one which I still hadn't understand but maybe that's a story for another time. Anyway, even without picking all the high mana cards, the battle was quite merciless. All of them stood to the end finishing of the opponent's quite mercilessly.