I don't know if something like this has been suggested before, it might sound a stupid idea tho.
The suggestion is simple: a new objective based on killing only one monster, which has various heads ( or something like this )
This monster would have special attributes, but my main idea is making it a caster monster only, and each head would use one type of charm, each having a special attribute.
For Example:
White head - use healing charms and heals him and other heads
Green head - use earth charms and have a small chance to cast earthquake(hit all slashers with shaking texts or something, really simple graphics)
Blue head - use ice charms and have a small chance to cast snow(make the slasher attack with half speed for 5 seconds)
Red head - use fire charms and have a small chance to burn(slasher will lose hp/sec for 20 seconds)
Black head - this would be the main head and would use a random charm each attack, having the chance to do all the abilities of the specific charm for that attack)
another idea that came is that Black head could only be damaged after all other heads are destroyed.
I didn't about a name or a reward for this so far.
Maybe another heads for wind and lightning charms(making them weaker)
Each head with a lifebar of course.
in this image, the only main change would be that black head would be in center
