A perfectly legitimate strategy during battle although not a brave one is to run away and let your minions take a beating if you need to heal up.
Only buy crystals when you need them to complete goals or in order to tade them for cool stuff.
Have characters complete joint tasks with other characters to level them up quickly.
When you need to kill Bungees use Chompers, Squash, Ice Shroom, Jalapeno, or an Umbrella Plant.
You will get killed quickly if you go charging head-on into a cluster of tanks. A better tactic is to flank and dodge out of cover picking off the enemy one by one.
Buying Charge unlocks a Duel mode which you can play against friends in hotseat multiplayer battles.