From what I've found, the matchup goes like this:
PK Freeze can only be used once space is created, it cannot be used to ESTABLISH space, once Marth is at Midrange you either use it while running away or STOP using it, because once Marth is at that range Run up > Shield will more than likely punish you with a hit or a grab. In the matchup you have to play so you don't get grabbed (As Lucas you should aim to never get grabbed, really, but there are some where you should ACTIVELY AVOID it) this means working on pressure so you don't get grabbed and not doing unsafe, unnecessary moves/movement because Marth has a wicked grab. Once WE get a solid hit Marth is pretty helpless because his moves once in the air don't do a very solid job of protecting him.
It feels like to me that the MU is one where if you play carefully, you win, because Marth ends up being very vulnerable once he gets popped up and we're very vulnerable when grabbed. These are just observations and opinions from playing the MU myself, I'm still not terribly good with Lucas. I'm sure another will gladly chime in though.