This Combo isn't that great. First, Volke with Stefan B is meh. They aren't even in the same chapters until endgame, which means they aren't guaranteed to have that B by 4E-3. So this 'awesomeness' is there for one map (lol @ 4E-4 and 4E-5). Yes, he could support Elincia or Tibarn as well, but this combo is still very limited.
Second, Gamble only works on player's phase, which means that he can KO one enemy per turn reliably (or two with a heron, but they have better things to do).