Instead of increasing the power of the monster cause there a higher lvl player
in the party extend his personal prot on that mob for longer duration. Either
based on lvls or exp worth. Another solution is the monster would contiune to
toss them from the room and scream at the higher lvl players.
You could make the eq dest depending on player level (apart from quest items).
That way highbies could still do the area/quest once for the explore. Might be
an idea to reduce the exp worth too, and give a message e.g. You would have
gained 150k for this kill, but because you're a no good fucker with too much
exp, you get NOTHING!