What about instead of clears (which is kinda ineffective) make the option to block players who haven't cleared the fight with let's say 10 new players.

It does the same thing. The players would have to finish the fight at least twice and push it morelikely thanks to their own skill as in groups with new players their contribution would be more important.

This way you could have farm parties with more experienced players and also motivate these players to help newbies to learn fights so that they could unlock the farm party option.