Once you get a second fighter, you will see a blue ribbon pop up next to one of the locked fighter boxes. Tap on it and it will bring up a list of available fighters. All of the other ones will have a purchase price shown next to them but the one that you just earned (usually by purchasing a box) will show up without a price, as being available to you. Select that to start that fighter on his or her road to the octagon.
Once you select that new fighter, that means that they will be right back at level one, except that thehir path will be different then the original fighter’s path. Typically, it will conform more to their weight class, if their weight class is different than their old one, or if you have a female fighter, it will consist of female fighters.
Switch back and forth between fighters if you get stuck on a tough stage. Doing this will allow you to use one fighter to build up silver and resources for the other fighter, and vice versa. Switch back and forth to train one or the other each time that you get boxes full of upgrades as well. Go through each move and see which moves are matches that provice 6x experience.
Otherwise, you have two options to grind if you get stuck on a stage and you run out of boost power to push you past the tough level. Go to the quick fights option and you will automatically match up against a fighter whose combined stat level is around equal to yours. Otherwise, go to one of the old fights and complete it again, with as many stars as possible (to facilitate more silver earnings).
Complete the quests if you want to earn a big silver bonus as well. If there is one that either takes too long to complete, or that you just don’t like, hit the reload-esque icon in order to replace it with a new quest. After that, the icon will go away for awhile, but after awhile it will come back. It costs nothing to skip a quest and replace it with a new quest, although you won’t get the quest rewards for it.
Time the use of your special moves (while standing up) to give the the best shot at landing. Wait until your character is in close. A good way to get them closer, more quickly, is to throw jabs to move them closer. Make a combo of jabs and then one or two jabs in, throw the special move. Landing the special punches and kicks correctly gives you a HUGE advantage in fights.