thrrr is 100% correct. You will notice that when your Guardian is up and running, different messages display when you accrue some points. I believe there is "Guardian Suppress", "Guardian Kill", and "Guardian Assist". The last one is what counts towards this challenge.
That being said, I think I've only seen one person with the Guardian Support title. I do have the other Guardian one with the dude getting melted and shot. It didn't take nearly as long as the Guardian Support one is taking. --- PSN: jovenal