My general strategy for overtaking any of the bases is to get a group bound to a key or two (12-24 attackers). I usually have the hero, a few ground attackers, a couple far attackers, and a couple siege weapons (catapults). Start with the beginning of the base... just select the entire group, click the attack button, and click an empty area on the ground. They'll attack all units in the area, then work on buildings.
Use your seige weapons to destroy guard towers first, then send in the troops. They'll pick out a target...
That's usually enough to take out any base. On that particular mission your life will be a whole lot easier if you attack the guys on the far right first. They don't have a very difficult base to defeat but they send out a lot of people.
Though, it depends on what you upgraded your hero with first. Your hero can help your units greatly when he attacks with them if you upgraded him correctly.
Except for that mission where you have to protect Kel'thun. Then there's the final mission of the game.... ugggh, that was a $@#$@#$@#$@#$@#.
The credit sequences are very funny though
Beat the final mission on hard and there's a little more to the credits than on normal.