Probably got already covered the one or other time.
If the enemie has level advantage, try to form the battlefield into your favor. Split one of the enemies apart and focus him down, lure them into a position, that gives you hight advantage, prepare the battlefield with barrels if necessary.
Or just go explore some where else or sneak behind them.
If you are in Fort Joy for example, you can get past the Fort itself and explore the swamp first do gain some exp and levels.