Hello!
Today I was playing Hammerwatch campaign with my paladin. I noticed that the combo nova blasts don't count towards the combo.
So when you have bought the "combo nova" upgrade and killed 10 monsters to get the combo mode on so the nova blasts start shooting, you don't get "combo 11!" even if you kill a monster with the combo nova blast. Only the kills you have made with your sword (or arrows) counts.
I don't know if it is intended to work that way or is it a bug but I think that you should get combo score by killing enemies with the nova blast.