Olimar isn't a horrible match-up for Lucas since they are similar characters in weight, smashes and recovery. You need to stay relatively far from him, since his forward smash is a powerful projectile, and he has the best grab in the game. Your grab is essentially the same length but has much much more down time. He's a character you need to approach from the air diagonally so as to avoid his ridiculously powerful smashes. He can't combo well on the ground so mistakes aren't punishable by damage, but can result in death since Lucas is rather light, Olimar on the other hand, is lighter. Once you see he's near 85% a well placed smash will kill him. Tilts are useless. Forward aerial is slightly longer ranged than his, but your neutral aerial is your best friend. Your dash isn't recommended b/c it's easy to see and Olimar can use his grab to get you. PK freeze is ok though since he has a slow dash speed. It isn't necessary to edge guard him for kills since he's light and you have great smashes (esp forward smash). He's not gonna edge guard you since he can't recover well, Olimars normally stand still. PK fire can be used to kill pikmen though (except reds) but spamming isn't necessary. If he's on platforms get under him and use up tilt since up smash is a little too slow. I'd use up smash if he's approaching you and at 75-80 for a quick kill since it's wide ranged, but you hafta time it right, and be weary or Olimar's amazing grab. Being underneath him is recommended in the air as well since his spike is short ranged and it's not horrible if you get hit by it. Careful about being next to him with your back to him, since back a aerial is a slow spike so I'd stay away from it.
So as a recap....don't grab, make sure you use a lot of aerials, be underneath him, watch out for his forward smash and grab, use PK attacks, and don't edge guard (but edge hogging is recommended)