When attacking, use a layer of bullet absorbers a little ways ahead of the back line to keep him from getting short range units to your longer range weapons.
When defending, take out their longer range weapons first, then destroy the other units at your leisure.

One exception. When attacking against MU\'s they are so fast you might need to target them and destroy them quickly before they get into the middle of your line.

Be creative, improvise, try different strategies. Have fun with it.  DON\'T be married to your city.  Your city is another weapon in warfare.  They cannot destroy any of your buildings if they attack sucessfully (howitzers, bunkers, AT guns, AA guns are the sole exceptions).   If they attack multiple times, you can take steps to prevent them from conquering your city.

Final words for beginners: 
When you are wondering what to do next, take a quick look through the missions, you might be very close to completing one.