Since I dont play the beta, I cannot say with any certainty if my advice is particularly applicible.
First, its probably never a good idea to let any AI sit and do nothing for any length of time. It may be expensive for either tech or for money, but its never a bad idea to pay off an AI to get involved in a war. If they are busy in their own wars, then they wont be as eager to take a shot at you.
Considering that, you may want to consider paying the other AI's to help you go after the AI your fighting. Chances are that if your becoming vulnerable, your opponent is likely to be worse off. The best time to do this would be when you start having a difficult time finding any of their surviving fleets. If you set the table for them, the other AI's should will jump in on a free dinner.
I would not be surprised if at least a few of the AI's are coded to let you or someone else do the heavy lifting.
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