I keep hearing bad stories about excruciating attempts to obtain Angolan visas by independent and trans Africa truck travellers attempting to transit.Despite stories that things are changing they stay the same!
I am not sure about Angola specifically. You might want to contact a passport and visa service as they would have the most up to date knowledge about this. Granted it does cost slightly more to do it this way, but knowing you have a professional that does this for a living handling your visa and dealing with any issues that arrive may be worth the extra cost.
Yes Angola is one of the toughest visas to get in Africa. Most are only able to get a 5 day transit visa, getting a 30 day is virtually impossible. It also depends on which country you apply for the visa, they are easier to get in certain countries. You also need a letter of invitation from an Angolan living in Angola if you apply for a 30 day visa, who will guarentee for any costs that may you may incur. Good luck in getting a visa.