It's great when your within his shield space and not directly above him, because his oos is a little slow (his grab is only 10 frames compared to the usual 6).
The trick is getting in in the first place, and avoiding usmash out of shield
Oh yeah, and if you've backed him up against a ledge from him trying to pivot grab you, your options become much better, because it makes it harder to allow access to his pivot grabs and you can push your offense, the only thing you really need to watch out for is his roll in that position, which is very baitable.
fair is your standard swatter for pikmin, to stop them from getting on you, and nair is if they're on you and you want to kill them off. Nair is also a decent approach option on shield, but you'll want to mix it up because he can just shield grab it.
oh yeah, imo avoid killing yellows if you can because they make the weakest smash attacks, try to concentrate on stray purples because those things hit haaaaard. btw I once tried to play keep away with an olimar, and it was really doable until I realized I wasn't killing any of his purples, and all of a sudden, he had like 4 lol.