The sacrificed creature's last known existence on the battlefield is checked to determine its power.

If you sacrifice an attacking or blocking creature during the declare blockers step, it won't deal combat damage. If you wait until the combat damage step, but that creature is dealt lethal damage, it'll be destroyed before you get a chance to sacrifice it.

Players can only respond once Fling has been cast and all its costs have been paid. No one can try and destroy the creature you sacrificed to prevent you from casting Fling.