On your first few turns of a duel send out creatures with a high amount of Mana and put it into your mana zone. (Preferably 5 or higher) When you have at least 6 cards in your mana zone put your smaller creatures in the mana zone. (5 or less mana) This way at the beginning you will be able to put out your small creatures and further into the duel you can put out big creatures safely.*

*This cheat does work. Tested by me and I won every duel so far and not lost a single one since I started!