The enemy gets stronger with time. Essentially there is a scaling accumulator variable that starts at 1 and 0.01 is added to it every second. If the accumulator value is at 1.5 for example, a spawned enemy will have at least 1 health and there is a 50% chance, the enemy will have 2 health. Following the same principle, if the scaling accumulator value is at 3.7, a spawned enemy will have at least 3 health, and 70% chance to have 4 health. So, the scaling accumulator value scales over time and all enemies scale with it, as well.
There is also a 'colossal chance' that has a chance to double the size and power of a particular enemy.
Concerning the enemy spawn locations, I just use 3 box colliders and only allow spawns inside their boundaries!
I hope my explanations were not that confusing :)