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{W}, Pay 1 life: Prevent the next 2 damage that would be dealt to target creature this turn.
{U}: Prevent the next 1 damage that would be dealt to target Cleric or Wizard creature this turn.
{B}, Put a -1/-1 counter on a creature you control: Prevent the next 2 damage that would be dealt to target player or planeswalker this turn.
If you activate Wandering Mage’s third ability, you put the -1/-1 counter on a creature you control as a cost. That means it happens when you activate the ability, not when it resolves. If paying the cost causes the only creature you control to have 0 toughness, it’s put into the graveyard before you can pay the cost again.
The second ability checks that the targeted creature is a Cleric or Wizard only at the time the ability is activated and at the time it resolves. It doesn’t check at the time the damage is actually prevented.
