Here's how I handle taking down the Koobo Duo mini boss in Stage 1. You basically want to kite them into nearly the same spot as much as possible to do staggering combos on both of them at the same time.
Otherwise, it's useful to always have one of them knocked down in case one decides to magnetize all the weapons off the floor. In which case, knock one down while the magnetizing Koobo completes their animation.
Grab the Koobo once the weapons start flying toward you and you want to time a throw to initiate your Invincibility Frames. And since Koobos are heavy, your character will prolong their throwing animation during the weapon flying phase.
