1. You could grab other ascension crystals. If you were to grab at least 3, you could ascend in 334 turns. However, the AIs will not like that you are ascending, so you would likely find yourself attacked by multiple AIs.

2. You could pay another AI (or several) to declare war in the said AI. That attacking AI(s) might get around to destroying the ascension starbases.

3. You can sit back and wait for the other AIs to take care of it on their own. They don't like other civs trying to ascend, so they will try to declare war on their own.

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Overall, ascension victory is one of those victories that is going to involve a lot of warfare. No one likes someone else trying to ascend, so they will try to stop it by force. The only way to avoid wars is to be strong enough that no one will dare attack you while you ascend. Arguably, if you are strong enough to win by ascension victory, you are probably strong enough to win by conquest victory.

I doubt that money could fix this issue, so research all the influence, invasion and weapons techs you can and go full-on war.

it is the only option.(try to get everyone ganged up on them too.(and do not forget defences)).

There is no multiplayer for this game. Not a whole lot of people were interested back when polls for it were up. So it is just AIs. You use metaverse to play a game, get a score, and post it online.

From what I understand, the game handles everything such as posting the score online and making sure you aren't cheating.

Let him ascend.  It takes 400 freaking turns.  Declare war and take out his ascension crystals 300 turns from now, assuming another AI hasn't done it for you.  

even if they take all five crystals, it would still take almost 200 turn for them to gain victory. at that point, you would have a fleet of armadas or maybe a planet full of golds. i have done this ascension victory thing, and the only way to do it is through absolute military dominance. if you bring the said AI's military down by giving more military ships to the opposing AI or go on the war path against the controlling AI yourself just to bring his military down, then he will be very unlikely to achieve the ascension victory. not to mention the said AI will not likely gain much of support from the other AI when they control the ascension starbase. they also get attacked more often which shifts other AI's attention from you. unless, you tick the other AI off as much as they do against the Ascension base controlling AI. some AIs are extremely passive, you really need to provoke them to do something before they become aggressive against those kind of thing.

Or less.

Anyway, the best course of action is to:

Step 1: Attempt to buy the bases.

Step 2: Declare war

Step 3: make everyone else declare war, even the minors.

Step 4: research the defense for the weapon branch the AI uses, and research another weapon type, (AI: MDs, You: Beams.)

Step 5: Conquer

Step 6: Repeat with every AI as they try.

I like twilight of the Arnor but I thought this was stupid I never play with it on.