My colossus attacked Rhysta and ignored the two minions present in the same area. Rhysta took no damage, but the way my colossus was on her was hilariousl. Wish I could make a little movie of it. If you have a mystic, you gotta try it.
Rhysta did move, sorta like a piece on a chessboard. Selestrus looks like a statue too, have not seen him move.
Since the colossus always attacks the most dangerous mob in the area, Rhysta seems to be more powerful than the minions or even Selestrus himself.
My colossus looked like he was enjoying himself. I couldn't make out Rhysta expression.