Grenades from a Grenade Launcher count as Guardians when shot at the finish line. Stand at the finish line with a Rally Banner or Warlock well with Lunas and shoot/spam grenades at the finish line. Even if you die it works. From RetroActive80 on Reddit: "In the race. Stand close to the finish line (don't cross) and spam grenade launcher rounds or telesto right on the finish line until your timer runs out and you die. It gives your team credit for a bunch of lap completions, gives you both triumphs and finishes the encounter. There's a hotfix today that some speculate may patch this, so you have about 40 minutes! Unless they don't patch it..."