Mac has a very specific focus when playing well: have control. From his bad recovery to his bad grabs, losing any type of stage advantage puts him at a seriously bad disadvantage. I suggest dodging more than shielding, as being grabbed or shield broken is pretty much either a stock or insanely high damage on a character like mac. Maintaining stage control is key, so keeping the opponent out is imperative. And despite being trash, Mac's aerials do exist and can extend combos and reset, mainly his neutral and down air. Combo'ing with his grabs is difficult, but his main go to option is down throw into up special, but if it's avoiding, expect some punish.
In my experience as Mac, it’s best to play defensively, and you need to figure out your opponent’ play style to use him most effectively. Get your opponent to come to you and react to the attacks thrown your way. This works best with other close range fighters like Ryu, but it works well against some of the sword fighters without projectiles too. In the case of projectile fighters like Samus and the Belmonts, you have to get in close. And Mac’s Smash Attack’s can do a lot of damage; it’s possible to take a stock in only four or five hits, and certain charges attacks can kill off the edge as early as 40-50%. Figure out your opponent play style, get in close and throw the Smash attacks where you think they’re going to move. Use his counter too; it’s one of the weaker ones but can be a little easier to hit.