The special tool to adjust the striker that Dev mentioned earlier is the Torx key size T-40. lend it or buy it.
Also try to put tiny amount of "rubber type grease" on the rubber seal outside the door frame that come into contact with the inner part of the body seal when you close the door.
Also put some grease on the slider that hit the striker and body to door hinge.
But don't expect it will easily be close with a push of a finger like Merc door. It's normal to push with a little force to close fd3s door. But if you have to slam it like mad bull then you have to adjust it. Adjust the striker with the windows rolled down.