A guide for easy levelling and coins:
Prerequisites: Tortoise unit, Meteor Staff. Preferably upgrades to the tortoise unit/defense upgrade.
Visit any boss screen and progress to the front of the screen. While killing the mobs at the front slowly, constantly summon tortoises. After approximately ten are summoned, stand directly behind them and break the fortress, forcing a boss mob to come out. The Aura, doubling the tortoises defence will keep them alive as you summon more. This allows the boss to summon many flunkies. Once a large number are present, use the Meteor Staff to knock them all out at once for a huge EXP and coin reward.
Unsatisfied with the sheer number of corpses you had to fire on level 10, I decided to do things a try shooting corpses at the bottom of the ball-wall. After approximately ten resets, I got the level done in one move.
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