Thank you for your moral support! :-)
My high-score I achieved exactly with Soul Drain but lately, when I enter the Shadow Realm, I am almost instantly surrounded by quick semi-invisible stuff and quickly killed and end up with 5-digits score. Also, I purchased two more main characters, with different abilities but I can't feel any big difference either.

I may give it a few more tries but at the moment more players find the most winning strategy, I bet the game will be "upgraded". ;-)