If a card you chose to exile leaves exile or somehow never moved to the exile zone in the first place, Lumbering Battlement doesn't get +2/+2 for that card.

Auras attached to the exiled creatures will be put into their owners' graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creatures will cease to exist. When the cards return to the battlefield, they will be new objects with no connection to the cards that were exiled.

If Lumbering Battlement leaves the battlefield before its triggered ability resolves, you can't exile any nontoken creatures.