Following along Shade's suggestion, walking shields or even running shields (by holding the direction you're running in then hold the shield button) work wonders for projectile spamming. It doesn't have to be a perfect shield, but the objective is to get close and not let him get away from you. Luigi's fireballs don't have the kind of power to break your shield easily, so you should be rushing in, block the projectile, then let go and continue the pressure. Luigi shouldn't have the kind of speed to run up and grab you if you use a SH FAir then back off and then followup with SB (The FAir assault tactic, in other words). They only time he will be able to shield grab you is if you stop infront of him. Never stay stationary enough to allow him the opportunity, and you have the movement in the air to poke and retreat before he can retaliate easily. Also if you tip with your FAir when he shields, it has a decent blockstun to prevent them from acting too quickly without them jumping themselves (and thus be open to your own attacks).
Hopefully this helps ya out dude
