In order to earn Blood Sigils and Coins, complete various quests and achievements. Save Blood Sigils until you have enough to get a least one Rare Pact, then use it. You can do this repeatedly. Do not use Blood Sigils to regenerate your energy or to restart a stage on which you just lost.
Elemental strengths
Your commanders and enemy commanders have elements that are stronger and weaker to each other. Blue is stronger against Red. Green is stronger against Blue. Red is stronger against Green. If you do not have a full stock of perfect matches in your party, create a match with the rarest or the highest level commanders you have available.
Improving your creatures
Spend gold to empower your creatures. You can improve the performance of your creatures by training them with gold. You also need to ascend them. This requires you to have duplicate versions of the same character. Once you have ascended them fully, they will automatically evolve and add another rarity level to them. Afterwards, they will be at level 1 again.