You dont have much time.... you realize this is the base your going to use as your base of operations, for future battles. Artillery is unavaible because you do not want to damage the structures too much. No, they will not leave. They are self sufficient to an extent. They could hold out for about 4-6 months without any supplies. And this base is also connected to other bases via underground tunnels... You have a little bit of time, but I personally think this is a more short scale battle. The leader is a brute(aka, very offensive), and the land around are other plateaus/large hills, but this is the largest one in the surrounding area.
That is me suggesting a new plan. but for your old plan.
IF you had the conditioned you wanted.(this is without the underground tunnels)
and their vehicle's wouldn't be draw out.
The Covie would bombard your troops from above with their tanks, and their banshees would of overwhelmed your air force by sheer numbers. The siege goes on, over months you slowly wear down their air force. After about 5 months they start probing for ways through your barriers. By this time they only have about 25-50 banshees. They would bombard you as best possible with their tanks, then from behind you would have a wave of unarmed grunts(bout 2000) to overrun anything back there and while these are easy to kill, their very vicious.... after that comes some brutes, bout 250 of them, and after that another wave of 5000 unarmed grunts, then a few hundred armed grunts. Your front is being bombarded with similar tactics. All in all, within 30 minutes of vicious fighting, your forced to call for evac.
Loses
all vehicals
and 3500 of your ODST's.
Enemy Loses
7,000 grunts
490 banshees
150 ghost
a few plasma turrets
and 2000 brutes
If your wondering how you got attacked from behind, they call for additional support. Without a strong base, your easily overrun.
Mission Failed---
A new plan with the new stuff I've told you, any more questions? and I know that your outnumbered, and outgunned, but this mission isnt impossible. Remember, they cant arm most of their grunts, and the only thing keeping the grunts in line are the brutes.
"Do you know what the chain of command is? Its the chain I go get and beat you with til you know whose in command round here!"-Jane, Firefly
Weapon of Choice- Unknown