Le Lone Wolf has a great solution, however, I'm adjusting it for single players. It is not as fast considering I'm not boosting with two friends but it only takes me between 2:00-2:30. More if I get bored and lazy.
You can boost online wins by going into multiplayer and choosing custom then 1v1. I also play on the Simple Map.
YES get assassins!!!
I also use Armadillos (Longhorns should work just fine though), T99s (T45s will not do, you need the upgraded firepower and armor/hit point combination T99s give you), and Money Makers.
During the match start up, make 5 Money Makers and put them in the four ports around your main base and on the one outpost you have. Next, upgrade the creep spawn time. This helps keep the computer busy while you're dropping assassins. After that build two armadillos.
Now the match starts.
Take your armadillos and drop them with UP on the D-Pad near the second outpost, the one towards the bottom of the screen after cresting the hill in front of your base. Dropping them with up on the D-Pad is essential as they fall into an effective defensive formation around the outpost once it is taken, saving you time on placing them yourself. Land and help your armadillos clear the base for your first set of creeps.
Once that is done, heal at the new outpost and queue up one T99 and four assassins as soon as you're able. Socket the T99 (between this and the armadillos your outpost should be well defended long enough for you to win), then grab the four assassins and fly along the bottom of the screen to the right. Then fly behind the main base. Hit LEFT on the D-Pad to drop the assassins behind the CPU's base.
DO NOT drop them above or below, otherwise they are spotted and destroyed by the T99s already set by the CPU. Sometimes your assassins get a death wish and roll out from behind the base to attack the sides, even after you drop them directly behind it. This can't be avoided as far as I know, but if someone figures out how let me know and I'll add it to the guide, giving you credit.
Once you have unloaded your assassins rinse and repeat. Two more trips, a total of twelve assassins, should easily bring down the CPU's base. You may need to add more due to the previously mentioned death wish syndrome, as well as the occasional smart play by the CPU, during which the enemy airmech will land behind its base and clear out the assassins. The last can be avoided if you drop your assassins quickly and do not land and engage the base yourself.
Good luck. If you think something should be added please comment and I'll try it. If it improves the time I'll add a note to this solution, once again giving credit where it is due.
Some changes since a recent patch:
1) Simple Map is now called Dust
2) AI Level can be chosen. I recommend going with Trainer or Basic. I have not noticed an increase to xp.
3) XP has decreased. Across the board the amount of XP per battle has gone down.
4) The space to drop assassins behind the base is much larger and the AI tends to fill it early with money makers and resource generators, so even when your assassins do roll out from behind the base they tend not to get popped so easily.
5) The map randomly changes after every win. Make sure you reset it.