Unrelenting Force can be purchased as part of the Paarthurnax's Roar deck or purchased alone for 50 soul gems.
Unrelenting Force should be used to unsummon weak cards with powerful abilities, like Lethal. At it's second level, it can be used to unsummon any card of any power and should be used to unsummon extremely powerful creatures. At it's highest level, Unrelenting Force should be used to clear the lane that has the most creatures and/or most powerful creatures. However, it is not advised to unsummon cards that have summon abilities, as these cards will have their summon ability activated twice.