Sprite Kit: Waiting to call a function until condition X is met - sprite-kit

Question

I'm working on a game (top-down shooter) and have run into a bit of a snag. Up to this point, I've spawned enemies with functions that just work with delays:
Wave One Function - Delay 3, spawn enemies //
Wave Two Function - Delay 6, spawn enemies
I do this because I haven't found a way to wait for all actions in a given function to complete before calling the next - as things stand, functionWaveOne calls functionWaveTwo, which calls functionWaveThree (etc).
This has worked until now. What's changed is I want two enemies to remain on-screen and until they're dead, I don't want the next wave to come. My initial solution to this was to have a running count of how many enemies died in wave four:
Detect collision -> Apply Damage -> Check if dead -> If yes, deadWaveFourEnemies++
Here's my problem: I have a primary weapon that's two parallel lasers, so they have potential to collide with an enemy at exactly the same time. This results in false positives, making the dead enemy count go higher than it should. I even tried adding an "am I alive" function to the enemy class, but to no avail.
Have any of you got a recommendation on how to either call functions in a better way or to get around these false positives? Thanks!
In case it helps:
if([enemySprite.name isEqual: #"shooter"]){
enemyShooter *enemyClass = (enemyShooter *)enemySprite;
itDied = [enemyClass doDamageWithAmount:secondaryWeaponDamage];
scoreIncrease = [enemyClass getScoreIncrease];
stillAlive = [enemyClass amIAlive];
}
[weaponSprite removeFromParent];
if(itDied){
[self increasePlayerScoreBy:scoreIncrease];
if(inWaveFour == 1 && stillAlive == 1){
waveFourKilled++;
NSLog(#"Seconday / Number killed: %i", waveFourKilled);
}
}