Treants are the most powerful faction at the moment but there are great units and combos in every faction available. Don't forget that the enemy can use those very same combos against you. Watch out! 

The main early game goal is to complete all the quests as quickly as possible. That gives you enormous amounts of gold to upgrade the party 

Some combos are clearly overpowered, for example the Treant Giant combo with the Emerald Belt. 

It's relatively easy to complete all the quests and farm gold. This gold is then used to spam rerolls to find the perfect combos you want to use. 

Got up to Trial Level 7. Might make a few changes to the starting party.