Since there is a bunch of shooting going on to start this one, the first order of business is to crush the attackers as quickly as possible since they are spotting for artillery that is probably placing a fair number of craters in your building. Be sure you move at least one sapper into position to destroy the enemy flyer.
But do not despair if you lose the building. You can easily rebuild it once the artillery shells stop falling, and it should only be a minor inconvenience.
Next, move a few units quickly to the north to intercept any new attackers and prevent them from getting within spotting range of your building. You need to bring new units onto the board quickly in order to go swiftly onto the offense. You will need to keep your guys moving as you advance on the enemy base to the north to prevent them from being chewed up by the artillery. If you find this annoying, consider dispatching the flyers to the north (possibly along with a couple of ground units loaded on the transport) on a search and destroy mission for that enemy artillery piece. You will have to manage this carefully though as the enemy engineers will wipe out your flyers if your can't keep them out of range.
Otherwise, just keep the flyers over the trees to minimize their visibility to ground attack (use the "Hold" command to keep them from wandering into trouble), and just keep advancing as rapidly as possible. Once in the enemy's back yard, use ground units to suppress the engineers so your fighters can nail the artillery piece. You can then bust the paneling off of his building at your leisure.
Remember, you must destroy the building and all engineers to prevent him from rebuilding his base, and to fulfill the victory conditions.
"We have found the enemy, and he is us!"