When you have priest and you need healing when you have no enemies, press defend, the priester's spell will hit your allies and heal them, use this strategy and you will mostly success
+ this comment so it can help people
there should be badges like i prefer : Easy badge 5 points : reach round 5 medium badge 15 points : reach round 15 medium badge 15 points : get all the upgrades Hard badge 30 points : kill - enemies. + if u agree