After hearing HG mention it, I decided to look up the speed kill video for Giganticus Lupicus and wow, its actually a fast fight. Here is their first video which is easier to see what they do with their feedback bubbles and reflect walls from the guardians. They used a well timed reflect to bounce the grub plagues back at the boss so no grubs to kill while meleeing the big guy. When he hits phase 2(he goes invulnerable for a few seconds when transitioning) they time a feedback bubble on him which burns a ton of its health.
Here is their latest video which is hard to see most of what they are doing but they did kill him a few seconds faster. All and all, a 2 minute fight. I don't see the other guardian dropping a reflect so I think a guardian and a mesmer can manage most reflects but a third wouldn't be a bad idea. Most classes have some kind of reflect. My mesmer has 2(or 3 if I count my warden). We should try this tonight if we are feeling up to it or just save it for like the weekend.