Phantasmal Fiend

Note that the wording "Effects that alter the creature's power alter its toughness instead, and vice versa, this turn" has been removed.

Effects that switch power and toughness are always applied last when determining a creature's power and toughness, even if other power/toughness-changing effects are created later in the turn. For example, if you activate Phantasmal Fiend's first ability, it will become 2/4. Then if you activate its second ability, it will become 4/2. Then if you activate its first ability again, it will become 3/3.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature may become lethal if you switch its power and toughness during that turn.