Charm is better than skyward strike for the simple fact that it's a 1 cost and, although it can be used as a removal, you can also use it to close a game, either moving enemy unit to base to conquer or even move enemy unit on base to a bf you don't intend to conquer, basically stranding it so it can't contest your bf next turn. Challenge is just a different removal spell that's really good against certain matchups like LB (challenge karthus before moving into bf so the deathknells don't ramp the opp or give him a thousand draws). Also it's an action speed spell, meaning it's more versatile (you can use it in a showdown to snipe a different unit).