I've been trying this level a few times and still have no idea, how to handle this. The AI is always reaching the bonus very fast, I can't stop him getting there, also it is not possible to reach his commander...
has anybody successfully completed this level and can give some advice??

You're doing the right thing! Just expand enough to get some income while also rushing into the middle - once you break the bonus, you should be able to take it over. Once you have it, reinforce it, and the game is basically won.

you're right! after a few tries I managed to crash his middle-bonus and finally I could get the whole middle-bonus and then it was easy... thank you!