How is he the most execution heavy? Besides his IA fireball I don't think his combos are that hard.
Well far starters, 90% of the cast is super easy to use. So by default, IA fireball alone automatically makes Zod one of the most execution heavy characters in the game. His only competition really is Flash with certain trait combos, Martain with some instant air teleport combos, Batman with some trait combos, and Zatanna with some float cancel combos.
But Zod actually has hard combos, like the ones listed above I posted if you just scroll up a few comments. Idk if you've ever used Zod before, but if you haven't fire up Injustice 1 and take Zod in training mode, summon trait, and try to do F213, trait slap, 123 trait grab. You will notice that you cannot just simply cancel normals into trait grab, so it is impossible to 123 trait grab without meeting certain requirements first. In order to make the trait grab connect after 123, you have to do a trait slap in the middle of the string. So basically you have to do 1243 into trait grab. But you have to press it really fast, if you do it too slow Zod will just stagger 12 and won't finish the full string. Alot of his trait combos work like this, which makes them unique and also makes them much harder than the rest of the cast's combos.
Also there are some combos that you can fit 3 Zod balls in one combo, but the combo is so long that the gravity is high and the character is falling fast, so the only way possible to do it by doing J2 canceled into Zod ball instantly almost as fast as possible, for all 3 Zod balls. For example 123, trait slap, 123, trait grab, charge close laser all the way, J2 slow Zod ball, J2 medium Zod ball, J2 fast Zod ball, B3. The first 123 pops your opponent up, then you still hit your opponent 5 additional times while they are airbourne before you even start hitting them with the Zod balls, so the low laser doesn't pop them up that high. That means all three of the J2 canceled to Zod ball all have to instant all 3 times, the first to get it started is especially the hardest.
I could go on and on with more examples, like how some combos you J2 Zod ball twice, and your opponent is too far away so you have to forward dash and instant J2 Zod ball to connect the third one. Or how you can do J2 Zod ball two times, then do J1 Zod ball to switch sides mid-combo which is really hard. But yeah simply just reading this comment you might not understand, but if you actually took Zod in training mode right now and tried to do all the things I talked about in this single post, you would undestand why Zod is the most execution-heavy character in the game