Baahsto destroyed mid action
I have to assume that the core rules are not changed in some way by this expansion as I cannot find the rulebook online. The core rulebook has this to say on conflict resolution:


  Over the course of a game, there are situations in which multiple effects would occur simultaneously or have competing effects. In all situations, card effects that happen simultaneously occur in the order in which the card entered play.


Since Baahsto entered play first you should play each hero's top card first from his effect. Once that has resolved you should go through the effects (if any) of the hero cards in the order they were played (turn order).

EDIT:
In short: I was wrong, there is now an errata. 

Relevant passage:


  Destroying a card before it’s finished
  
  When a card is destroyed before it can finish its effect, any remaining effect is cancelled.
  
  For example, Ra has his “Flame Barrier” out, and a villain target deals
  all hero targets damage. The players decide to target Ra first, and Ra
  immediately retaliates, dealing enough damage to destroy the
  attacker. With the attacking target gone, there is nothing to continue
  dealing damage to the rest of the hero targets.


So destroying Baahsto stops the remaining hero(es) from drawing a card from their deck.