If the countdown is at 400, then the AI has most likely two crystals. The Ascension victory requires 1000 points, and each crystal provides one point per turn.
As for possible actions, well, there aren't many. You could try to find and capture the other three crystals. If you are quick enough, you may beat the AI to the Ascension victory (but it will be close). However, the other races won't take kindly to your attempt to ascend, and will most likely declare war soon.
You could also try to go for the Tech victory. 400 turns should be enough to do that. Just make sure, that the AI doesn't capture another crystal.
Lastly, there is always war. However, you don't have to fight yourself. You can try to bribe another race to go to war against the one trying to ascend. Before you do that, though, place a combat ship next to the ascension starbase, and then give it to the race you want to bribe. With any luck, they will destroy the starbase for you.
Well, I hope this helps.
Edit: ...and ninja'd.