I have WL7
Don't fall for the easy mode, it may seem like you're doing good, but once it goes away, difficulty will increase by 5x.
Have high quality fighters and upgrade them, buy ammo at 280-320 to arm them. Don't upgrade ammo.
DO THIS QUICKLY BEFORE THEY CAN REACT:
Send AA units (such as fighters or Off AA turrets) to attack the defenses of undefended counties. Destroy all enemy interceptors that way. Then, send in occupation units and spec ops to paint the country side. Make sure to keep an eye on the enemy war index, once you do 30 damage points in painting, bomb the capital. Then bomb all the corps you can. If it still isn't yours, bomb the cities and take it over.
Limit yourself to WL3, and KEEP YOUR LEADER NATION IN SECURED MODE!
Then you have 2 options:
KEEP IT: invest the money and grow it slowly. Don't put more money into it. You might wanna RAID a few times before going to keep one. Because it takes some 1.7T for a taken over nation to rebuild and start to grow.
RAID IT: Go to direct trading and transfer all the dominated country's cash to your account. Sell the valuable stuff (production plants, aircraft fuel, jewelry,luxury goods, wine, and spices) to your country, and transfer the money into your account that it receives. Exchange professionals with them many times until you have a huge labor balance and can build as many corps as you like. Shut down all the country that you took over's corps. Wait a month, it will have money from all the scrap and ruins recycled. Transfer that. The country you took over is now in shambles and will auto-rebuild within' 1 year back to full strength. Plus, the fact that it will have to make new, weak, uncompetitive corps will vastly improve the world economy. Remove the country that you took over, raided, and shambled from your account. MAKE SURE THAT YOU DO NOT REMOVE YOUR OWN! REMOVE THE RAIDED C3. CHECK SOME 5 TIMES THE NAME AND NOT SOMEWHERE SOMEONE WILL BE DUMB AND RANDOMLY PRESS THINGS ON YOUR SCREEN.