personally i feel like lucas is one of Marth's worst MUs. it's very difficult to beat his neutral game and he can be difficult to kill if he's in tether range while recovering.
your best options are to reach mid range with DDing and WDing and then wait for an attack. lucas' like to nair on shield to begin combos so spot dodging is not an option. you have to shield grab once the attack is over. the risk here is that he can grab you, which YOU NEEEED TO DI AWAY FROM so that he can't follow up with a combo. if lucas is below you, you're F-ed, so you need to have very calculated movement. the whole MU is dependent on you getting grabs. it's a lot like playing against Fox. Lucas can be juggled in the air fairly well, and will likely use his second jump to evade your combos. it's a very hot/cold playstyle that you have to work into this MU. chain grab with JCGs when you get the chance and finish off with f-smashes to get him off-stage. try and hit him away from his tether recovery range so you can edge hog or guard the up b.
that's all i've got from my experiences. just don't end up above him and try and move in for grabs and tippers. DI away as much as possible, unless you have an opportunity to DI into the ground and tech away.
gl