If Marth jumps at you, shield is pretty much safe. He might try to tomahawk you, but if he gets close enough to you or the ground, you should anticipate this and avoid it. All but the lowest aerials will be very, very unsafe on shield. CC beats a lot of non-tipper moves until higher percents. Ideally, you should focus on DDing vs ground game, the biggest thing to look out for and avoid being him dtilting your dash back. Getting close in his face or moving past him tends to work well and can cause him to royzone, as can close jabs. Something else to remember, Marth has poor active frames on most moves, so you can spotdodge most attacks and punish, just beware of Marth waiting or DDing instead of attacking.
Once he gets a grab, you can actually just die. You can try to DI to make the followup ambiguous and mix it up each time, and at around 20 percent if you do no DI you can force Marth to pivot. Once you get towards utilt percents, DI far out then try to SDI into Marth and DI as much as possible for the utilt hit that sends you behind to make the followup as hard as possible. When Marth goes for uair to setup a tipper, try to SDI either up, or away and possibly downward (preferably the latter). If away, DI in anticipation of a tipper finish, this DI makes it harder to continue the combo. Be ready to tech, not towards Marth if he's not going to hit you. SDI up should be for much higher percentages and when you're already displaced horizontally.
DI tipper fairs out unless it will kill you or put you farther offstage horizontally than vertically, and treat non-tipper fairs more like standard combo moves. When it isn't going to cause death, especially if it won't cause knockdown, DI down and away. Do survival DI when it's sending you far enough for this to KO. Obviously this is simplified, but it's easier to avoid followups from tipper fair the higher you are relative to Marth, and things can vary based on Marth's motion as he's hitting you.
DI fthrow at 0 down and away, buffer spotdodge or roll if he goes for immediate regrab. Otherwise, you can try dashing towards him dashing towards, or jabbing.