To earn this achievement you must win 750 fights total. The fastest way to earn this is to play the first level over and over again. Each fight lasts 15-30 seconds as you will soon be able to kill the earth dragon from the first level In only one hit. You must spend one energy per fight and one energy is regained every 25 minutes. You can upgrade the ancient portal to hold more energy by spending gems.
Also, winning duels count towards your total fights. These cost one "red energy" and each red energy is regained every 40 minutes.
This achievement is easy, but time consuming.
Note: each time you gain an experience level the ancient portal energy is fully restored.