For the Meerslug, just charge at it. Every time you see it. Even when it does the sandtrap attack, you can charge your Pikimn at it's mouth and hurt it. The Scornet Maestro only targets your ACTIVE captain - Have one Captain sit on one end of the arena with no Pikmin, and once the Maestro starts to attack, switch and charge at it with Pinks. Saddest excuse for a boss in the series.