Huddersfield head into the final round of the season needing a victory after being thrashed 48-6 by league leaders Saints.
Luke Thompson and Morgan Knowles celebrated their Great Britain call-ups with two tries each as Saints ran riot against the understrength Giants.
London’s victory means the Giants must defeat Catalans next week to secure their Super League status.
Tries from Luke Thompson, Dom Peyroux. Regan Grace and Morgan Knowles ensured a 24-0 half-time lead for Saints, who lost Mark Percival in the first-half to a concussion.
Michael Lawrence opened the scoring for the Giants in the second half, but order was soon restored as Saints ran in five tries, with Thompson and Knowles scoring again along with further tries for Tommy Makinson, Kyle Amor and Jonny Lomax.
Saints: Coote, Makinson, Naiqama, Percival, Grace, Fages, Lomax, Walmsley, Roby, Thompson, Taia, Peyroux, Knowles. Subs: McCarthy-Scarsbrook, Amor, Ashworth, Bentley.
Giants: McIntosh, McGillvary, Mellor, Jake Wardle, L Senior, Russell, Holmes, Clough, O’Brien, Matagi, Ta’ai, Hewitt, Lawrence. Subs: Leeming, Ikahihifo, Kirby, Wilson.