Have you tried using the "Search" function? It really helps since most questions here have been asked and answered before. Try searching there first next time
Nevertheless, when the opponent gets dodge happy, it may help to dash dance just outside of their range (dashing back and forth on the control stick as fast as possible) which makes them not know when you are going to attack. If you do this for a second, you can see the rhythm of which they air dodge. When you know this, it's a lot easier to time a quick run up and grab or forward smash.
Also, practice your spacing: be able to consistently stay just far enough away from her (Samus) so her jabs and smashes can't hit you but you can hit her. It's tough and takes a lot of skill, but it is essential for any good Marth player.