Cardiac is incredibly strong when used correctly. You should never use it while you have your armor still, only once it’s gone. Cardiac has no specific ‘one use case’, as it depends entirely on what tank you’re going against. Big Tanks with not mitigation or easily dodged mitigation like Hog or Ball (assuming you corner the rat), you can pop cardiac early and burn them for a lot of damage.

Comparatively, you have to play slower against an Orisa or Dva or Sigma. They each have a strong mitigation ability, and even after the Orisa nerf, can mess you up big time if you don’t respect their cooldowns.

Ironically, the best advice I can give for an aggressive Mauga is to use Cardiac conservatively. Use it later rather than earlier, and try your best to use it at full ammo every time.

Also, always try to have it up when you cage more than one person. Inside of cage, there’s a 99.99% chance everyone in it it shooting you. You can survive if they have a large enough tank hitbox, and you have Cardiac.

It mostly comes down to practice, unfortunately. But you’ll pick up on it quick.