Holy hell, how does that happen? Looks like rockets and grenades, but what rockets and grenades?

Gjallarhorn. I explained it earlier.

Clearly a team has already gone into the portals and come back out.

Apparently, the rockets also get a massive damage bonus when Time's Vengeance is active. Couple with tracking sub-munitions of the Gjallarhorn and you get many, many high-damage, sure-hit impacts in a short space of time.

I don't have that weapon :(

Yeah, they clearly had to do the run for Time's Vengeance, but from the moment that kicked off, it was darn quick to get his health from 100 to 0. It's definitely a team using nothing but Gjallarhorns, as I was telling someone last night that I had long-ago noticed my Gjallarhorn does much more damage than my sniper rifles due to each sub-munition getting the damage boost, and pumping 10 rockets into the poor guy packs a serious punch.