If Shape Stealer's new toughness is less than any damage on Shape Stealer, Shape Stealer will be destroyed as a state-based action once the triggered ability resolves.

The other creature's power and toughness are determined as the triggered ability resolves.

Once the ability resolves, changing the other creature's power or toughness doesn't affect Shape Stealer.

If the other creature has bushido, whether Shape Stealer gets the bushido bonus depends on whose turn it is. If Shape Stealer is blocking, it won't get the attacker's bushido bonus. If the Shape Stealer is being blocked, the bushido ability will resolve just before Shape Stealer determines what the creature's power and toughness are.

This ability triggers once for each creature blocked by or blocking Shape Stealer. If multiple creatures block it, Shape Stealer's power and toughness will change for each one in succession. The first trigger put on the stack will be the last to resolve, so that will set Shape Stealer's final power and toughness.