If you want a more challenging AI, there are custom maps with AI that afaik doesn't cheat and are insanely good. Search Fyn AI or google sc2 custom AI, they are just custom maps with modified scripts running and might provide a challenge for you and your mate. If you're beating the very hard AI, why not jump on the ladder? It is programmed so that you should be winning 50% of your games and always challenging you.
just f10-n a heap of games and get in the league you feel you should be in and go from there. it's not like losses matter. Though perhaps being in the higher leagues can be some good practice for you, depends on what you take from each game I suppose. Playing SC2 because you want to win EVERY game is a bad idea unless you're masters or GM.
The actual playing of your placement match has very little to do with where you'll get placed. during the league locks before the new season starts, you are expected to play as many games as you can and come season 3, whereever your MMR is is where you will get placed, the placement match is just a trigger for that to happen.
I play on SEA and just before the lock i was promoted to silver, and after winning my placement match I got placed in gold, and then I lost 3 games in a row and won my 4th and i wasn't impressed with my play :[ too tired.
I'll do my NA placement tomorrow, I was gold and i've been beating platinums so we'll see how I go.
I still haven't quite worked out the new maps, before my placement I looked at them all by observing some AI 1v1 matches, just to learn the layout properly. I don't know if i like or dislike them yet. Some of them have some pretty odd design choices.