I don't write guides very often because there are plenty of helpful people on here who usually beat me to it ;-)
This game has only been finished by a fraction of people and it's easy to see why. I finished it today and was the 48th player to do so. As there are no guides for this achievement, I will try my best to explain...
Ok so to get to the general's league you have to progress through the previous leagues leading up to it. Select league in the Xbox Live menu and then search for a game. You have to FINISH 6 games to complete a season. YOU DO NOT HAVE TO WIN 6 GAMES. After each game is completed you can see your league stats on the right hand side. With each game finished your points are added. All you need to do is to earn sufficient points to get promoted (usually 80).
Bear in mind that if you eliminate a player you receive 5 points for each elimination whether you win the game or not. If you quit out of a game it does NOT count towards your league stats and basically, the game does not register the loss. Whether this will be patched at some point I do not know, but for now this can be taken advantage off if for example you were on your sixth league game and you need a win to achieve promotion to the next league. For winning a game you receive 20 points and each elimination is added to your total. I managed to get promoted in each league with on average 3 wins. Once you have reached the promotion threshold you can just relax and enjoy the game knowing that it does not affect your score if you lose and you need only finish the remaining games to complete the season.
On the point of finishing games, if people rage quit, they will be replaced by AI. DO NOT WORRY about this. It can be a good thing as the AI is usually pretty stupid and easy to beat. If you eliminate an AI which was a player it still counts towards the eliminate 50 players achievement and also counts as a elimination in the league which will net you 5 points. Even if all players leave apart from you, the game continues with AI and if you win IT ALL STILL COUNTS towards the season.
If you are really struggling finding a game another technique you can use to speed things up is search for a game, which if you cannot join someone else's results in opening your own game. Wait for one player to join. If they press 'ready' press 'A' to start the game. This will launch a 1 v 1 between you with the remaining 3 teams serving as neutral players which do not attack. I found this much quicker and finished a season in under 2 hours using this method. Again, if the player rage quits which they often do, happy days because its just you and with one AI player to beat. You actually don't even have to beat them, you just need to capture 30 territories and you win the game. It is also worth noting that if you eliminate the neutral players they net you 5 points each for an elimination. This really is the easiest and quickest way to finish a league. I had a couple of 1 v 1's where I collected 40 points for winning the game and 4 eliminations.
To begin with matchmaking was terrible but it seems to be getting progressively better as time goes on and the online community increases. One of the hardest challenges as somebody has already mentioned in a guide for a lower tier league achievement, is actually finding a game. Every now and again you will get lucky and find yourself in a full lobby ready to go. But now usually if you are patient you will find a game most of the time. If not refer to my 1v1 method above.
I really hope this helps people as I can see not many have got it yet. Please comment to let me know what you think. I will alter the solution if necessary and am open to suggestions. If you must vote negative, please explain why so I can fix the issue.
Thanks for reading. Happy achievement hunting
EDIT 1: Generals league relegation 0-49 , stationary 50-79 and win the league 80+points
so not 100 needed. Thanks to TNelson1995 for this info.