Luigi all the way! just because he is a low tear, doesnt mean he sucks, look at ka-master, Rofl, Ultraluigi and other great luigis, if u main him i suggest u stick with him!
now since i convinced u to stay with luigi ;D here is some help:
samus: you can Ftilt or tornado h-e-r missles very easily, i prefer tornado cuz not only do you disrupt the missles but you also can get very close to h-e-r that way, and when ur in close range, samus has nothin on u, just get h-e-r off the stage with whatever means necessary, and edgeguardin samus is pretty easy
shiek: if she spams needles, just WD -> shield, and when u get close to her, up throw or up smash and when she is in the air she is pretty easy to combo, play a bit more defensive tho because shiek can get nasty if you let her
marth: you can look up "ROFL tells you how to beat marth. " and it tells u all the details, however if u dont feel like searchin for it, just upthrow at low percentage -> into an aerial, and at mid percentage downthrow into an aerial, do lots of Ftilts, and if he is fast/tries to be fast, just kinda shield, or play defensive and punish him when he makes a mistake
EDIT: thx Binx