I do agree... after seeing a lot of comments on this issue, king of blood does feel a little underwelming. We're doing a rebalancing to make the demon become viable on wave 15ish with the right build. Also, the exploder unit is getting stronger.
At this moment, I guess the mangler + towers build is the best option for the king of blood, but it's not really obvious or easy to hit it.
Thanks for the feedback, though!