Aqua Tail or Earthquake depending on your team's needs, Aqua Tail gets Volcarona like you showed, but won't get Heatran unless 3 turns, however Earthquake gets Heatran, even though it wouldn't take out Volcarona anytime soon. So EQ or Aqua Tail depending on your team's needs. So I agree with your last statement.
Why would you run Aqua Tail or Earthquake for Volcarona / Heatran, respectively, when Hidden Power [Rock] or Hidden Power [Ground] accomplishes that while also damaging them so much more? Aqua Tail / Earthquake doesn't handle any other threats in particular that Goodra either a) wants to be dealing with or b) can't touch already.