Train your cards to their maximum level before combining them. By doing this, they will get better ratings when leveled up. This will give you an advantage over opponents that did not train their players to maximum level before turning those cards pro.

Level your best players at about the same rate. Do not max out your top player as fast as possible, as it will make your lineup one sided.

Use your enhance cards only on Pro cards, and only those of higher rarity.

Use Common cards as trainer fodder once you get Uncommon rarity players at each position.