When you have started a new game, get to chapter 5(keep Natasha or Moulder alive for this chapter) . Once you are there, defeat everyone (after getting Natasha to talk to Joshua) except for the boss. Because the boss(Saar)does not move, you may do whatever you please. I suggest to go to the arena with Ross, Joshua, or other characters that need exp. Whilst you are at the arena, your players may get damaged, which is where Natasha and Moulder come in. Keep fighting over and over again in the arena, even when your characters are all lvl 20.