So on the second image I posted in the main post, there is an Array node which is connected to Number Of Zombies. This will give each zombie a number upon spawning.
On the enemy itself I added a check so that whenever the enemy is killed, it will remove itself from the array.
Then, there is an constant event called Wave Check running to check whenever there is zero enemies with a number in the level, and if true, it will move on to the next level.
Here is the Wave Check custom event.
