Activate Abilities at 0?

[Pleeb]

He would have had to do it in response to you declaring him as an attacker, but if he's doing it after declaring no blockers, then he dies. Right? Because he doesn't get priority to respond again until after Papa declares attacks until after the combat step... Right!?

The only time he has priority to respond is after the declare attackers.. Right!? D:

I'm just now learning about how damage isn't on a stack thing and it's blowing up my brain 😭

To expand on what giggleflat said:  players receive priority after blockers are declared. As long as your response hits the stack before damage is actually dealt, you should be fine.