Sorry I was at school and tired. Lemme look at this.
Obligatory
I'm gonna have to correct you on that. Nightmare has two different attacks in game that he uses, as he uses one where he does, in fact, scream, though it's dififcult to interpret this as him having to scream but instead just a preference, but besides that he also has a killscreen where's it's completely silent and you only hear a high pitched beep. Of course I don't remember much about FNaF lore so maybe that's not a canonical attack he can use, but still.
Technically speaking Nightmare uses teleportation a lot to get from one hallway of the house to another, so it's not too strange to say he'd use it to confuse Grant on where he's coming from, and if he teleports behind Grant and screams, Grant would hardly have enough time to respond in the first place, as all it takes is a single bite to the skull which could be easily secured. Nightmare does bite while screaming I believe so that does mean he can do both at the same time, likely due to a voice box.
Now you're overpowering statement, sure, that's fair enough, but what can he do after overpowering the robot? Let go of Nightmare with one hand and he's creamed, because he's going to get stabbed, or otherwise could just be bitten straight up due to the fact you'd also have to restrain his head. Grant doesn't have three arms to fight back.
And when you talk about throwing him over his shoulder I'm...very much iffy about that. Nightmare likely weighs a lot, a couple hundred pounds at least? Considering he is an animatronic. I understand that's grant has peak human lifting strength but I'm not sure if he can easily flip something that weighs a couple hundred pounds or more over his shoulder before that thing just bites him or stabs him, and it's gonna be hard to find a weakness against sometimhing that won't give you the time to do so, as I'm sure if Grant walks away from the animatronic far enough Nightmare is just gonna teleport to him,