Q.: What's the most effective and efficient way to do Bat's bats move (D^J), when a new phase in Survival Mode starts?
A.: Position Bat in the upper or lower third of the screen and get close, but not too close (maybe 1.5 dash distances) to the opponents. This way the opponents might get hit by the bats from two sides and they tend to pick you as their next target, making some of them not defend against your bats.
Q.: What's the most effective way to use Bat's bats?
A.: Bring an opponent into DoP, then do not grab them, but do the bats move instead (D^J). If you're standing more or less in the same spot as the opponent, all of the bats will hit them and it'll might deal a massive amount of damage, depending on how many bats are spawned. Of course this is only the most efficient, if you really need that one opponent gone. Or if you just want to pay some back ...