Once Undercover Butler's ability triggers, it doesn't matter what happens to players' life totals before the ability resolves. Undercover Butler won't be able to be blocked this turn even if the defending player doesn't have the most life and isn't tied for most life as the ability resolves.

Undercover Butler's ability doesn't trigger if it attacks a planeswalker or battle, no matter what the life total of the planeswalker's controller or battle's protector is.