Lens began the day in third place, two points above Monaco in fourth, and they never looked back after Openda opened the scoring in the ninth minute at the Stade Bollaert-Delelis.
Openda netted again in the 16th minute before setting up Adrien Thomasson to make it 3-0 just before the hour mark.
It was the perfect way for Lens to bounce back after a 3-1 defeat away to leaders Paris Saint-Germain last weekend effectively ended their chances of winning the title.
Lens, who last appeared in the Champions League in the 2002/03 season, moved back up into second place.
Only the top two in Ligue 1 qualify automatically for the Champions League, with the team finishing third entering in the third qualifying round and fourth place going into the Europa League.