In this game, unlike in Blood Brothers, you can’t sacrifice troops to level up other troops, only to evolve and that requires two of the same troop. If you want to strengthen your troops, play until you level them up, thus increasing their stats. You can play old stages as often as you like, so do that if you are stuck on a stage.
Also, if you’re stuck, change up your mix of troops and your formation. If you normally go with the offensive/defensive mix, try the all-melee or all-ranged formation, or the full-on defensive mix. If you normally play with the circle formation, try others, such as the skein or the line formation.
You have a chance to earn a large amount of experience and coins at the end of a battle, when you can either destroy the opposition or let them go. Save your Skill Procs for this point. Then choose “destroY” and hit the enemy fighters with every “skill proc” that you have, so that you can finish this portion as quickly as possible.
If you really want to maximize the statistics of your evolved cards, hold off on evolving them for as long as possible. Each evolved card retains 5% of the stats of the two combined cards (in addition to what it already gains), so the higher you let the experience levels build, the higher the stats will be after the evolution, even with the level dropping back down to 1.
When picking a level, hit the red dot next to the battle name in order to see all of the level’s terrain statistics. Rougher terrain decreases movement speed, meaning that rougher terrain gives a much bigger advantage to your ranged troops and your flying troops. Ranged troops can shoot from safety, and flying troops’ movement isn’t affected by obstacles.