I'm having a total blast with Middle-Earth: Shadow of War, but I'm at that infamous endgame now where you have to basically fight a prolonged war and the average level of your enemies jumps up dramatically. This was clearly designed to entice people into buying loot crates to help even the odds, which is definitely shitty, but fortunately I found a good way to level up without having to resort to loot boxes, so if you are playing or have played this game and gave up around this part because the grind was so bad, here's my advice:
Lose your forts on purpose.
Seriously, do it. Defending your forts requires a ton of resources and you'll be going through your spoils of war like water trying to buy all these upgrades to fend off attackers who are ten levels higher than you. However, if you lose the fort first then all you have to do to move on to the next battle is reclaim it. This is much better than trying to defend for two reasons. Firstly, you can still lower the fort's level by doing missions to kill off individual defending captains. It's much easier to attack a fort with a 120 rating than defend from attackers with a 380 rating. On top of that, you get 3x as much XP for capturing a fort as you do for defending it. You're guaranteed at least one level up for every fort you capture, maybe even two. Is it still a grind? Yeah, a bit, but much less of one. I'm halfway through after just a few hours using this method. It's possible the last few stages will jump up again and force me to find another way to grind, but I still went from level 40 to 45 after just a few battles.