What I do is play a demigod that I know can kill a computer demigod 1v1 100% of the time. Usually this means, I have to have some way of interrupting their TP scrolls or simply do so much damage that they can't get away fast enough. This means that I usually play Oak/Sedna/Unclean Beast. I then try to get some early kills by exploiting the AI's behavior. Before the update, it was pretty easy to lure them into towers and then interrupting their TP scroll when they tried to get away. I haven't tried since the update so I don't know if they're smarter now. Once, I've gotten a few kills everything becomes easier and I'm around equal level with the other side's demigods. Since the computer just randomly decides which skills and items to buy, I have an advantage as long as I'm not 2-3 levels below them. At this point, I forget about my allies and attempt to accomplish the objective singlehandedly. You will never be able to stop your idiot allies from feeding the computers but you can make up for it by hunting down the computers every chance you get. I ignore the citadel updates, there's nothing there that will solve the problem of your allies sucking.

Another trick, you don't have to win every game but you do have to win around half of them. And you can restart for free if you get off to a bad start by simply quitting and starting the round again. The game type doesn't change but the map does and some maps are easier than others.