Mettaton is a mid game costing card with great durability, much like it has in Undertale. No matter what the attack of the opponent, Mettaton will only receive one damage, (exceptions include abilities that directly change health or destroy cards as opposed to deal damage, e.g. 'Same Fate' or 'Headshot') this makes it incredibly hard to defeat, and with boosted stats from Snowdrake or from 'Force of Nature', Mettaton can become a great asset and obstacle on the board.
Best used with the Kindness soul type, and applying 'Taunt' to it via 'Protection' or the Glad Dummy. Using Effect Cards or abilities that deal x amount of damage that is not randomly split amongst enemies (e.g. Punishment) will still do only 1 damage to Mettaton.