defense phase. Because this has no way of impacting the board (removing cards), it is dangerous to include in your deck. However, there are not many cards that can allow you to know you are going to win when you attack for lethal damage. This can do exactly that when paired with Swamp Witch, Slime Rain, or even Sneak Attack.
It is often a good idea not to play it until you are ready to win, as a Fresh Force could ruin all your plans if you play Mist Machine too early.
Strong against: Control decks
Weak against: Aggro decks