This solution involves getting into a glitched Online public free roam. You will know you are in one when you go to whistle your horse, it does not come. The next step is to go in the options menu and put your online aiming to expert (This is because you get extra XP if you go in expert aiming). Next, go to any gang hideout on the map. You have to be the only one at the hideout(Basically, if the hideout is flashing, don't go to it!) As soon as you enter a gang hideout, it will end. You will get anywhere from 200-400 XP(Depending on which hideout you go to. Twin Rocks gives 400XP). When it ends, press left on the D-pad to replay. Repeat this over and over until you reach lvl 50. Now all you need to for now is constantly press left for however long you need to. It look me about 3 hours to get from 25 to 50. That's pretty much it.
Oh, and I use TA daily so comment or message me if you are having trouble following this,
Still works as of July 2016 with backwards compatibility