Referencing to my discussion with shroud about experience gain, he showed me a really bad logic in this game. You can easy target 50 levels above monster and train fast your levels on them.

In WOW they build in a cap. So it's becoming more and more harder to kill monsters very high above you.

Here is my theoretically cap, build in damage calculation.

playerdamage
roll (rnd 100%)

if monster level>playerlevel (used skill)
  if (attack is magic and roll is >5%) or (attack is non magic and roll is >10%)
    playerdamage = playerdamage - playerdamage / 100 * 5 * (monsterlevel-playerlevel)
    if (playerdamage < 0) playerdamage = 0

if monster level<playerlevel
  if (attack is magic and is roll is >10%) or (attack is non magic and roll is >5%)
    playerdamage = playerdamage + playerdamage / 100 * 5 * (playerlevel-monsterlevel)

monster damage
roll (rnd 100%)

if (rool>10%)
  if monster level>playerlevel
    monsterdamage = monsterdamage + monsterdamage / 100 * 5 * (monsterlevel-playerlevel)
 
  if monsterlevel<playerlevel
    monsterdamage = monsterdamage - monsterdamage / 100 * 5 * (monsterlevel-playerlevel)
    if monsterdamage < 0 monsterdamage = 0

this means.

Player damage is decreased by 5% each level difference to monster
But player have always a kind of critchance to inflict original damage, this chance is 5% for magic attacks and 10% for non magic attacks

This also works on grey monsters, except the critchance is more a chance to fail, so monsters have a chance of 10% to dodge this damage boost on magic attacks, and a chance of 5% to dodge this on non magic attacks

And this also works on inflicted monsterdamage, we can boost or reduce damage there to.

...

in gamebalancing this feels like you fighting with a 10 level higher you inflict only 50% except you crit, you fighting with 20 level higher you can't harm except you crit.

This is only a basic idea and a logic. You must think about this also compared to your end content dungeons. (no one currently reach.  )

And players can still kill level 50 above, but it's hard work and no more efficiency for leveling.