In fighting Olimar, I've had a little difficulty. He tends to hide out and spam throws at you to rack up percentage and goad you in to approaching, at which point his downsmash comes out of nowhere and sends you flying.
Tips:
Try to dodge, shield, or reflect his Pikmin. Reflect is especially good, as it then racks up damage on HIM, and he won't be expecting it.
Edgeguard and edgehog. One of Olimar's biggest weaknesses is his recovery.
Spam your own projectiles to kill thrown pikmin and rack up your own damage. PK fire works nicely for this.
Try to get him above you. Olimar's only downwards move in midair is his rather weak and short-ranged dair, making him easily juggled by Uairs, Utilts, PK thunder/freeze, and the fatal upsmash.
Olimar needs to be on the ground not at an edge to refill his pikmin supply. Try to keep pressure on him and pick off those thrown at you when opportune. Don't give him time to rearrange his pikmin, refresh his load, plan out what he is doing. However, he will have time to pluck six in his starting invincibility, so don't try to harass him then.