Nissa - Vital Force, does the land that becomes a creature can get destroyed?
Yes, it will be destroyed by having sufficient damage on it. Also, Terror ('Destroy target creature.') will work just fine.

The text:


  'It is still a land'


signifies that it can still tap for mana and that it is affected by spells and abilities that target lands (ie: 'Destroy target land').

Once it is removed from the battlefield, the card becomes just a land card. Since lands are permanents, you can target them with Nissa's -3 ability.