I don't think he loses those matchups because I use Shazam wrong, I think he loses those matchups because I destroy every Shazam I play with Doomsday and grundy. I feel like they are too dominate up close and Shazam can't zone so he is forced to fight them up close.... and he doesn't do so well lol. Vs grundy every time he touches you, you lose 50% of your life. Vs Doomsday, his footsies and rushdown are too dominate and he can blow up all of your wakes up on reaction.
When I fight Shazam with Doomsday, every time I knock Shazam down I wait to see what wakeup attack he does. If he teleports towards me, I punish his teleport with a combo. If he teleports away from me, I punish it with venom. If he stands there and does nothing, I MB Venom for frame advantage and start my pressure. Shazam seems helpless