For the phantoms idea I got a solid base and growth tree(phantom units).
Growth TreeBuilders(50 hp) -> can turn into builders+(75 hp) at s2
Builders+ can attack but normals can't
Builders -> can turn into Small attackers(50 hp) (half human size)
Small attackers -> can turn into Small attacker+(100 or 75 hp) at s3
All small attackers(and+type) can turn into Middle attackers(150 hp)
Middle attackers can turn into Middle attackers+(200 hp) at s2 or Large Attacker(500 hp) at s3
Middle attackers+ can fly so they can kill flying people.
But here is the catch you need to suck the health out of humans or aliens(high damage). So when you get 200 hp you can grow into a middle attacker+ but when your health goes lower than 200 you ungrow to normal middle attacker.
Phantoms Building
1. Phantom Flooder
-makes a pool of health sucking goo and slow you down actives when person or alien passes it
2. Phantom Wall (hard to make)
-a wall that hurts alot if you step thought, phantom can go thought without a worries.
3. Phantom Grabber
-noodle like thing attacks you holds on(

+

)
4. Gaia
-the center makes spirits, build points (ei

or

for phantoms)
5. Phantom Spawner
- I think you will this one