Why are the dandori battles so hard for me?

Maybe try just watching your opponent for a round to get an idea of how to manage Pikmin for that level. Just use the time to watch, then try to replicate the next round.

If you struggle finding Pikmin, there is an item you can get that calls all idle Pikmin to a location. That is helpful to makes sure all your Pikmin are either with you or actively doing something.

You may also be able to open the map and find Pikmin without time passing. The drone can be for that too as time pauses when you activate it. That way you can find your Pikmin without your opponent getting ahead while you're standing still.