After defeating Al Gore, this quest will automatically become available. Head to the church and find Al Gore disguised as ManBearPig hiding in the church yard. Hit it to fight it in battle.
Don't expect to deal that much damage since MBP has a exceptionally high armor rating. Your only way of defeating it is by primarily stacking up Bleeding damage. It has a skill that will allow it to regenerate so Burning and Grossed Out damage are not enough. Bleeding seems to deal the most damage per turn that it can outpace the regenerated health. Use your abilities in this battle as well though PP will become a concern. Heal up per turn as necessary.