I assume light doesn't matter for picking which monster to place in your discard pile?
Correct. Light only matters when you are attacking, not using a dungeon effect.
Do you get your normal attack in addition to this ability?
Yep. It's a dungeon ability, so it's resolved before you choose which monster to attack. So you could go into the dungeon, destroy one food, refill the Dungeon Hall, then choose a monster to attack.