100k upwards is good. A multiplier of x50 is generally essential, it turns normal rocks into gem-rocks.
I'm trying to add another hard enemy to the end game at the moment. Based on the "+" from Ending - it starts off in a wall and will push you to death. Hit it (1000pts) and it breaks down to a 4 sided spike you can only kill with a bomb (2000pts). Currently it's better to run away than engage so I have to add another monster - a friendly rock that's also a bomb. Aiming for high risk / reward, hopefully will make time outs a bit harder but not impossible.