From what I've seen, the key to Atomic bases is Uni upgrades, strike fighter planes and their correct usage, and Tactical Helicopters to replace healers so you can ignore sniper towers.
As far as army loadout goes, you can go with the tried and true field howitzer heavy army with mercs to serve as heavies. I've also seen people including 7-8 bazookas to kill walls quickly along with shooters and howies.