The AI gets mercs in a war, then after the war disbands regular units as it can't afford it in peace time, then decides it needs regular units, then it again can't afford and disbands seems to be the biggest issue. In another thread about this someone said that it was infinitely better debt wise if you just disable mercs.....
Want to wreck the AI for a minimum of 50 years (and maim them for the rest of the game if you do it early as they won't be developing an economy, as if you look at countries who didn't they will be building buildings)? Make them hire mercs. They will be useless for at least the next 20 years. I've seen it repeatedly, in debt and no manpower because instead of firing the mercs or lowering maintenance/mothballing/ect... it builds regular units as soon as it has the manpower to do so (as it should, it is smart to not necessarily disband mercs that can't be rehired for 20 years until you are sure you can defend yourself), and then it deletes units as it can't afford it. And it KEEPS doing it! So when you see big debt, you may want to also check the manpower, I have seen it break that cycle eventually but 1. the debt is still there, 2. it hasn't developed it's economy and others have.
So, the right ally is one who didn't hire mercs early in the game (or recently, unless it built an economy that can afford it). And the countries that will under-perform and betray you are the ones that did.
Just completed a Big Blue Blob because I destroyed England's standing army in the first war, he never had more than 5000 manpower again through 1497 when I got the achievement. And that was while fighting no one else and having about a 10 year truce between each of our 4 wars. I got him to hire mercs after the first war, and then he just spent the rest of the time hiring units and disbanding. I'll take the achievement, but.....I'd like at least a competent AI to play a full game with.