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Knock out six enemies with a single Explodz explosion in region 4.

The 6th level in region 4 is best for this. There's an Explodz box on the ground with glass above it, and six enemies surrounding it, perfect for this achievement. Simply hit a ball down to break the glass, then a second to blow up the Explodz box. All six enemies will get knocked out. Here's a video for reference.