Some thoughts:
- I've never had issues with my computer allies not carrying their weight throughout an entire tournament, at least not consistantly - some games they under perform, other games they don't. The key is to always get more favor than anyone else, so as long as you are your team's top performer, you'll be okay.
I would get currency for your team ASAP, but otherwise let the upgrades slack until you can go straight for cats and giants. This depends on the game mode, obviously. Big maps mean not getting unit upgrades until late, small maps mean push hard and fast, focusing on holding flags and pushing to giants.
How are you upgrading yourself? Health stacking, especially as Sedna vs AI, can make you an unstoppable force.
If your AI is sucking it up, feel free to tail them and use them as an HP sponge while you do the damage.
I would recommend playing UB, as his build is straightforward and is a very strong 1v1 demigod late game. A straightforward build will let you focus on improving your meta game tactics.
Lastly, play with some friends over Ventrilo! You'll start seeing ganking opportunities, techniques, and other tactics you can apply to single player.
Nothing is an instant win - especially with random allies and opponents you can't talk to - so don't stress and have fun. 