I might not have said this before (just joking), but Yang Kai should take one of those portals to the demon realm and continue the process of having Gun Gun devour the place. Sure, he can try to single-handed solve all the demon invasions, but in reality that will only give the demons more time to do whatever is the big picture they are doing. But if the demons lose their demon realm and thus all those portals, what are they going to do in the long run? Not to mention the great emperors still need to be located.