I just watched the Beast Of War lets play video where Charybdis was used. According to how they played Charybdis and how Leo explained how active powers work. It does not count as a Simple or complex action.So you can activate Charybdis use your 2 simple actions then activate Groundswell power.
So same logic applies to Medusa's Petrify?... she could move, petrify, then attack (preventing petrified target from using talents or retaliating)...