"Weak demons"??

And of course, you need brimstone and wood to upgrade them.

Yeah, that's the biggest problem :D. You can easily find brimstone (and not just brimstone) to upgrade succubi, but you can't find enough for the dogs... Just a little info - on some maps, succubi don't upgrade at all, and are only used as a stepping stone to lords, while dogs are upgraded in the first week.