My tactics after struggling with meta,
Usually defensively, LEARN HOW TO SHIELD!!!! Learn every single concept of using the shield. Shield dashing to jabs for instance. Also, another example would be to shield dash to a grab in a way that you extend your grab by sliding a bit with the shield dash and not pressing the a button too quickly (basically do this instead of dash grabbing since it sucks)and fthrow, you can do about three chain grabs with this, especially if you do this after a good fireball spam. Sorry if this isn't clear, if it isn't let me know.
RAR your bair.
WOP tactics with bair for spacing, definitely keep your distance from him.
FIREBALL SPAM LIKE YOUR LIFE DEPENDS ON IT!!!! USE IT TO APPROACH, ALSO USE IT INBETWEEN YOUR COMBO SETS. ESPECIALLY USE IT TO RECOVER AND TO INTERUPT META. IF ANYTHING, YOUR FIREBALLS ARE META'S WEAKNESS, HE'S TOO SLOW TO DODGE THEM ALL!!
Keep mario in motion, move around a lot!!! Roll away from meta, short hop air dogde to shield, play around with fludd in mid air while DIing, mess around with the dair to move about, anything to make yourself less upredictable. However, do not use a mid air cape stall to play mind games with meta unless you feel the need to do so, meta will fair you to bits if you do...
Bair to flare, single jab to ftilt or dsmash, should be your standard combos, don't make it predictable though. Usmash is good for kills too, stutter step with the usmash if meta short hops often...
Use a fully charged flood wisely to create space, for example, if meta uses his fsmash spray em...
Meta sucks off the edge, hit him with a meteor and its cookies!!! I've pulled of a horizontal fair into a vertical fair in one match before and it was like complete pawnage. Instant meta KO!!
Just remember that it is possible for mario to be meta, although you'll probably have a grudge against sakurai for making another unfair match up in brawl. keep playing against meta and improve your defensive/evasive play with mario, GOOD LUCK...