According to this calc, The giant Unicron avatars that Unicron can manifests are 8-A, Megatron himself cannot directly fight them because of their size differences and had to resort to hit and run tactics. His tactics were obviously successful and his basic energy beam attacks were already shown to be capable of damaging the avatars like how he destroyed the knee of one them in the episode in where he fought 3 of them so he could stall for more time for a space bridge to open for him and the Autobots awhile ago. The punch that shattered a Unicron head wasn't necessary 8-A itself but there are many times in the wiki where we accept minor damages and them holding their own againts said characters who were stronger than them like MCU Ironman scaling to base MCU Thanos for giving him a good fight and even making him bleed. And for this case Megatron straight up can just keep running and gunning all 3 collosal Unicron Avatars if he so choosed.
Megatron's fists are still enough to damage them a lot though. Despite the rocky nature, he could have easily destroyed all 3 of them if aimed for the heads. If his punches can shatter their heads (despite only being mde of rock) followed by with the rest of the bodies beginning to crumble from head to toe, he could likely also do considerable damage to the other parts of the avatars' bodies with the same strike. Also laser beams in the show tend to be quite lackluster for some reason (despite Megatron killing an avatar earlier with lasers to the face). And if we consider making people bleed, giving them a good fight, and shattering their heads from their bodies to be usually 9-C or 9-B for humans usually, what's wrong with scaling Megatron with the 8-A avatars? The rocks he used for the avatars are surely stronger than human flesh and bone.
I don't think he could, otherwise, he would have done that, he was problaby looking for an opening to attack. And yes, he could do considerable damage to other parts of his body with that strike, but that won't change his rating. Hell, you could argue that he can't do that normally, since he needed a bit of momentum to do that punch
Well we could do a small change at least, swap out the speed ratings of the Bayverse and Prime continuity characters to Subsonic+ for combat speed and reactions due to missile dodging (characters like Max Payne are rated for reasons such as that since RPG rockets are like Mach 0.9 iirc) and still let them keep them their Subsonic movement speed on vehicle modes and probably even Supersonic to Supersonic+ speeds for aircraft moded people like Soundwave and Megatron.