Also, I recommend playing on stages with low ceilings, such as Corneria. The entire stage of Corneria is basically one gigantic slope. The ground is uneven, which reduces Marth's attack range. This also reduces Olimar's smash range, because the Pikmin are not traveling on a perfectly flat surface, so they end up dropping down the slope and not being as effective, or they go against the slope and do not travel very far. However, Olimar's grabs treat the stage as if it was a flat plane. When grabbing, Olimar's Pikmin travel a surprisingly far distance up and down the stage, so it's very easy to grab Marth on Corneria. Also, since the ceiling on this map is so low - I would say the lowest out of all of the stages - it is easier for Olimar to kill Marth. Olimar can even kill Marth using up-airs, juggling Marth into the ceiling. And, of course, Olimar's up-smash is even more effective on this stage.
Olimar has the advantage on Corneria.