If he tries to side-b spam you, just do a lot of f and utilts to kill them.
Avoid being grabbed at low percentages, as most Olimar with grab, dthrow you, quick dash to up smash, then uair juggle you to infinite and beyond. <.< and if you are grabbed, just mash that airdodge and DI away from him and hope he doesn't hit you.
Whatever you do, space yourself further then his Fsmash, and just wait to punish him.
Gimp whenever he's off-stage, especially if he has no purple pikmin.
Be wary of using laggy moves due to the SA frames he gets from down-B (meaning if your facing a good olimar, your chances of spiking are pretty low except MAYBE your dair, but your fair is a bad choice unless your facing an olimar who does not abuse those SA frames)
If he ever has a purple pikmin next in line, DO NOT GET ABOVE HIM if your at 100%+ (somewhere around that, maybe 80? lol) as you will get killed.
Whenever his pikmin are desynched, kill them off and don't let him get grounded because he plucks new ones fast. (speaking if your not facing an Unomar or Duomin (1-2 pikmin users like myself)
Avoid using side-B because I do believe he can easily grab you out of that and punish.
thats all I can really help