If you are not a hardcore completionist, I recommend stopping at 950gs. In fact, if you are a hardcore completionist I would almost suggest skipping this game entirely. I really love DW, I really love playing it, but I know that if DW wasn't a kind of game I truly enjoyed I would not even attempt this achievement.
It is not a difficult achievement, it is a time consuming one. Allow me to break it down. There are 41 characters, who need a total of 2009 levels gained for this achievement. Let's assume, that doing the other 950 achievement points you got 4 characters to level 50 and 20 characters to level 20, which knocks 576 off the total needed. This leaves 1433 levels to gain. Let's assume you're using two controllers. Let's assume an average of two levels gained, per character, per scenario. Let's say you can complete a scenario (Loading screens, prep, fighting) every 5 minutes. 12 scenarios an hour, makes 48 levels an hour. Doing the math, this leaves you at just under 30 hours of grinding levels at absolute maximum efficiency. Doing 360 Castle He Fei speed runs in a row is not something one will likely be able to manage without severe psychological trauma.
Keep in mind this figure does not include time spent farming horses or weapons, which will take longer. Any of your characters you're playing who do NOT have good weapons or horses will invariably increase the time needed to complete this achievement. Also note that from what I've gathered getting 4 characters to 50 and 20 characters to 20 is a GENEROUS assumption during the process of getting to 950gs. I would assume that the other 950gs could be done with as little as one character at level 50 and 16 other characters at level 15 or so.
Please see the Experience, Weapons, Horses, and Chaos Mode FAQ for more information on getting this achievement done as quickly as possible.