Shrink Ray + Beam for Meter
Splitter + Buzzsaw for Micrometer
Hex + Lightning for Millimeter makes the best combo!
That's an inherently flawed combination by nature, hex and shrink ray both do flat damage whereas splitter does percentage based damage, and since balls have more health based on how small they are you want the higher/percentage based damage for the higher health balls. Otherwise I'd say splitter on micro and hex on meter, just cuz hex kinda sucks