He is talking about the The (in)famous OS/CS/KS guardian combo.
If it can run its course it can kill a vit character that has a reasonable damage drop (i.e. not 80% DD) in a single sequence. But that combo sequence is engraved in stone and can be interrupted really easily. A quick stun in the middle of it and your damage peters out as your opponents walks away... or kills you. Guardians take a fair bit of skill and timing to play because they can't simply run around hitting folks with 20k regular damage whacks at the rate of 3-4 per second or nuke folks with a 200K damage whammy.