1.The giant monster that's destroying everything is your old friend that you thought had previously died, and him destroying everything is curing a plague at the same time.
2.The final boss is actually a vacuum cleaner
3. Your strongest opponent yet has been hiding under the stage that you and other opponents fight on the whole time, and it leaving has caused the stage to have one good side, and one evil side.
All three are different games.
Infamous 2, LittleBigPlanet 2, and SSB4.