I feel like the mana isnt the issue. The card itself isn't too strong, so increasing mana would just put a bandaid on the situation.
this obviously isn't a new argument, but the card itself does not promote any kind of healthy interactions. The problem is that the text can cause so many completely random outcomes where neither player has any influence, and the opponent has no ability at all to guess what card they could have. It just surprises them when they play it at the perfect time, and there's is no way that you could have guessed they got an ice block or a pyroblast or whatever, so you can't play around it. Most the time the card doesn't change the course of the game much, but when it does it just feels so bad as the receiving player because there is literally nothing you could have done about it.