But what denominates weakest? Since

is treated as 0 before given a value, it will always be the "weakest" on the field. It's a/d will always be 0/0

Try this: "The attack and defense of this card are each equal to the lowest attack of all other memes on the field multiplied by 1.5."

Or if you prefer: "The attack of this card is equal to the lowest attack among all other memes on the field multiplied by 1.5. The defense of this card is equal to the lowest defense among all other memes on the field multiplied by 1.5."