In the Epic card game, when is event damage resolved during the attack?
From the Complete Rules:


  5.16.3 If a champion has taken damage equal to or exceeding its defense in a single turn, it is immediately broken.


Thus the defending player will take no damage from the attack. It looks like technically there is still a "Choose Blockers" step even if all attackers are dead, but you can just block with nothing. (Description of Battle Phase: Rulebook p12, or Complete Rules 4.4)