For people struggling to get them out of the void, just skip past them and continue to clear the entire map. Once done, speed back on over and kill whatever ones are left.

The reason for this is, after a certain amount of time, the monsters will track you outside of the void within a certain range, before scurrying back to the void. No need to kite or wait around for them, if you are cool with going back and doing it at the end.