This card’s effect to “…end the Battle Phase.” causes play to enter the End Step of the Battle Phase. The Venom Counter is placed on the monster that attacked, then any other effects that happen “at the end of the Battle Phase” are resolved.
If you activate "Ambush Fangs", and the attacking monster's attack is negated by a card chained to "Ambush Fangs ("Negate Attack", etc.), you cannot place a Venom Counter on the monster that attacked.