I think the mission levels is more of a guide only, they don't level up if you go into a mission 'over levelled' there were ones I found tougher than I expected also. The best I can say is while you may be at a higher level, make sure you update your carrying capacity etc and modify your weapons as you receive them as well. You could be level 50 with all the skills available, but if you have the most basic weapons, then you will still find it a bit of a slog. As you progress through the game, you will see that Glinthawks are a piece of piss in grand scheme of things. (Smash it with fire)