It's not the monsters' levels that are the problem

The answer to the question is in the question itself. Ohnoes monsters my level are to weak and I can't train on higher level mobs cause they're stronger? Lolwut?
Nothing is stopping you from training on higher level mobs (xept maybe at higher level the Time Temple quests >_<). The misses make up for more hits/kill, making them more challenging.

Of course, you lose KB and other mob control (no more combos generated on an Aran, so you build finishers slower)

And the acc stat becomes completely silly, because by spending points in a primary stat other than STR, you guarantee you'll always have 2-3 times as much accuracy as you need before accuracy even affects the formula.

And there's a hard cap of +20 levels - it's impossible to train on anything more than 20 levels above you. I have several times in the past done that, because as you said, it's a challenge. Even Warriors could do it - my Aran trained on mobs up to 23 levels higher than itself and it was completely dexless.