You can do this in different ways:
for example, you can check how many enemy class left each time when you destroy/kill enemy. Use Get All Actors of Class -> length and if it returns 0 that means you defeated all enemies.
This will work if you destroy enemy object when they are defeated. If you do not destroy them, you can manually count them (for example, in your foreach loop count only dead enemies - where Boolean isDead is true).
Just create some generic function that will perform this check.