I'm getting very close to completing this too.
Try having the two sentries to the left of the drill (if you are facing away from it) in the 'lake', pointing at the drill and the cliff top where scorpions hang out. They will concentrate on the scorpions. Place your gun turrent covering the 13th hive through to the ladder. You will target everything that comes at you and the sentries.
When rhino comes, throw a flare at it as it attacks your sentries. Then empty your turret into it whilst waiting for the aliens to gather around the flare. Then use your soflam to nuke the rhino and aliens.
You are correct that using your points for purchases at the right time is critical. The game can change in an instant and usually for the worst.