hi,
just out of curiosity, would it be feasible to give every creep an individual score value?
So your score would instantly increase when killing a creep. The score could be lowered on how long it takes you to kill it too f(deathtime - spawntime). An incremental bonus might be more complicated though e.g. if you'd kill a creep from the next wave before finishing the current wave, it would get a lower bonus.
Guess this would be more resource intensive, so not sure if it is a good idea, but i would fix frogs.
What do you think?