Just keep in mind, that you will be able to beat Nightmare AI in 5v2 if you are good. That means you +1 AI ally vs 5 AI's.
Nightmare is the "hardest" difficulty level.
I myself can kill nightmare AI's in 5v1, but lose eventually due to the AI destroying towers and attacking the citadel. Even if it means their death lol. AI is really stupid, and it is good for training or testing builds only.
I suggest you play with friends on LAN.
Get some more friends of yours to buy the game, and play 2v2's. Fun guaranteed.
Edit: However, Frogboy said, that there are many people who have bought the game, and played it Singleplayer exclusively. So it is somewhat based on your opinion, likes and dislikes about the game.