It wasn't that difficult to begin with.
at level 60 on the psx version I was only concerned with Elibdus, and not much else (Sure the odd Tiamet could accidentally ruin my day, but only if I went looking for them).
lvl 55 is about where you need to be if you want to beat the game with just a smidgen of difficulty, and thats if you have your eyes closed. As the game is called, all you need is the right tactics, and even handicaps can be overcome (except maybe the battle at Riovanes, 1v1 Wiegraf and Ramza, that one can get annoying).
Also, overpowered isn't necessarily over leveled. Overpowered is just having the right skillsets on people. by lvl 60 (hell, the first battle in the chapter you get Steel even) I had a 97 Brave and Blade Grasp for all my characters. You're essentially unhittable for just about every physical attack in the game, and coupled with 4/5 people on my team having a 03 faith? Magic didn't do much. Before I even got to the final set of battles the only thing that could hurt/hinder me were breath attacks and certain status effects (and even then, ribbons negated that for most of my team). That was before 60.
A dark knight by itself isn't that rigged, not from what I've seen. It was so much easier to rig things with a duel wielding monk, and thats SO much easier to get.