Invasion of Earth (as seen by the invading aliens)

Commander Alien UÚUÚ (CA): Alright staff. We are in orbit around planet 32G-ZT7. Looks very habitable, but there are alien life forms on it that have to be removed first.
XK'TTYQ! What has your research shown?
Researcher Alien XK'TTYQ (RA): They are bipedal. Internal Skeleton. Omnivores. Intelligent. They are very territorial and will fight to the death to defend or aquire new resources.
CA: Sounds like trouble. Alright. What's our invasion plan? Don't forget. We used up almost all our resources to get here. Whatever plan we decide on has to work ok? We don't get a do over. Captain Z'Z'Z'Z'YQQ? How's the nuclear option?
Captain of the Scoutship Alien Z'Z'Z'Z'YQQ (CSA): We can nuke them from orbit with our heavy weapons no problem, but not without destroying the ecosystem.
CA: We can't have that. We need those resources. We don't have the time to restore the whole damn planet. So no hammer. What about the scalpel. PFFFF'FF! Tell me my options.
Groundtroup Leader Alien PFFFF'FF! (LA): 100 soldiers have been woken from cryo sleep so far. We have the technological advantage, but the natives outnumber us 80 million to 1.
CA: Yes but videos show not all of them are soldiers.
LA: Even if we assume only 1% of will oppose us that still leaves us vastly outnumbered. And one lucky hit can kill us even with our advanced weapons and armor. They have been practising on the native animals and each other for millenia.
CA: Can we wake all the troops?
LA: Yes. 10000 are available. But we will run of food almost instantly. We must wake them and invade basically on the same day and hope to find enough food on the ground.
CA: Sounds risky. Any other ideas?
RA: A bioweapon. We could take one of the plagues that already exist on their planet and manufacure lethal bioweapon.
CA: What are the risks?
RA: For us? None. Our biologies are non compatible. It would just take a while till it has spread.
CA: Sounds like our best option. Proceed!

Later....

CA: Report. How is our bioweapon doing?
RA: Not good. It has only killed 99% of the natives.
CA: 99% sounds pretty good to me.
RA: That's still 80 million left.
CA: Oh right. What happened?
RA: They became immune. Just like that.
CA: Can you adapt the weapon?
RA: I'll try.

Later again...

CA: Progress report XK'TTYQ!
RA: Bad. Very bad. We engineered 3 more bioweapons. But they killed less and less of the population.
CA: How is that possible?
RA: Well, we killed out those with a weak immune system in the first wave. The young, the old and the already sick or injured. The remaining people are in general the healthiest and strongest of the population.
CA: How many are left?
RA: Um. Lets see. We wiped 99% on the first wave, 70% the second time, then 40% and now 10%. Around 13 million people.
CA: Still so many? Damn. I guess we have no choice now. We have to land and fight on the ground and hope for the best.

A little later....

LA: Ground Troop Leader to Commander! The estimations were way off. They are opposing us pretty much 100%. Under external threat they banded together to fight us. We have no chance. We're getting slaughtered. Aaaaaah!
CA: Damn this planet and their inhabitants. They were the death of us all.