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Me and my friends have spent many hours trying to get the answer to this question. We've done the math, and I'm almost positive we can give an answer that is about as close to the right one as anyone can possibly give.
The ranking system works on an experience bar for all levels under level 16. The magic number to level is 3. Now when I say that, lets say your in a 4 vs. 4 map. On your side you have a level 4, level 8, level 12, and level 8. Your teams sum then would be 32, so in order for everyone on your team to gain rank you would need the other teams sum to be 35 of greater. If they have you beat by 3 points, then you will rank (fill your experience bar).
Additionally, there seems to be a max to how many levels you can gain: 3 being the most. The game will not let you rank up any more then 3 at once, all the way to lvl 12. Once you're level 12, the most you can gain at once is 1 level per game.
Now to get to level 16, it takes 10 games. If you're playing a level 16 with a fresh account (level 1), and the level 16 losses to you 10 times, you will become level 16. Also, that level 16 will not derank (for a very, very long time). At level 16 you can lose 40 games in a row and still not derank (even if your losing to level 1s).
Once at level 16, this gets complicated...you can no longer use this magic number of 3. For each faction there is one level 20, two level 19s, three level 18s, and four level 17s. That's all there can be. 16 and under are unlimited, 17+ are limited. Now this is where the math, becomes unknown. However we still crunched the numbers (based on leader-boards), and there are only two possibilities on how this could work. After level 17 you get a new experience bar where wins have no cap and can't be simply gained. If you win a match you get points (and the level which you win against gives you X amount of points in this bar). This bar is a number that can be added to and subtracted from. So let's say you get 1600 points if u beat a level 16, and you only get 1500 points if you beat a level 15, 800 if you beat a level 8, 400 is you beat a level 4, etc. Now lets say the same principle applies to losses. You lose 1600 if you lose to a level 16, 1500 if you lose to a level 15, etc. Your win / loss ratio will have no merit, which is exactly what it's like - a level 20 can be 160-14, and a level can be 100-0. Win to loss ratio doesn't apply (because 100-0 is way better then 160-14 percentage wise), so why is he level 20 if he doesnt hold the best record? Its because his mathematical "bar" has the largest number out of all the other bars. 1600+1600+800+900-700-400-200+1600+1500+1300 ( 7-3 ) is better then 1300+1100+400+300+300+400-1600+200+200+600+600+300+100 (11-1). Even though his ratio is worse, at a mathematical viewpoint player 1 is better then player 2, which is why he is the lvl 20.
That's the math, and that's how its done. Now I said there were 2 possibilities. And sadly to say the second is exactly like the first, with an X factor. I say I believe your ratio doesn't really apply, however it very well could (but is unlikely). Whether or not your ratio is divided or multiplied or if you take the whole ratio and turn it in a percent and do something with it in order to determine who is the best I cant tell; crunching the numbers on all the players on the leaderboards to figure that out would be exhausting, and I really just don't feel like doing it. I think it would be a waste of time, because I strongly believe in the first possibility.
I was able to make a level 18 Werm, and a level 18PE. By making level 16s from level 8s and 10s, I did the math. Took me 84 games with the level 18 Werm, and only 25 games to make the 18 PE using the exact same rank in players (beating nothing but level 16s, never losing a match, from level 1 up). Thus, the X factor between how to get from level 16 to 17 to 18 is how many players are listed, and how far they are on the point spread. Werms been around a lot longer then PE which is why I was able to do 25 with PE games what took me 84 with Werm. There's that many more players with larger mathematical sums.
Hope this helps everyone out there who have been guessing for all these years.