I don't know whether Volkare is harder than a megalopolis, but I'm planning on trying a solo game with the final city set to a level 22 megalopolis at some point. Level 11 cities are a bit harder to conquer than the usual level 5 and level 8, but not that hard. I just usually take more wounds.
Volkare usually looks terrifying at the beginning of a game, but I've found him to be pretty manageable by the time I actually settle down to fighting him. It might take a few combats, but it's not as hard as I'd expect. I still haven't played against him solo though, so I'll have to see how that works out, and I need to try the hardest difficulty some time.