I actually really enjoy difficulty slopes. It's one of the things I love about many single player RPGs. You're fighting a boss and it's difficult but totally doable. But as you get further and further, he fights harder and more desperate, and by the end it's a mad dash to beat him before you just completely run out of steam. The damage output is immense, you're running out of consumables, maybe there's a time/turn limit, etc. Fights should get significantly harder as you near the end, because the enemy should be pulling out all the stops to prevent his end.