Look at the button on the side of the controller here. I bet if you got close enough to one of those scourging crystals, you could use the controller to bring its shield down. Then it would simply be a matter of destroying the thing before its shield goes back up.
What do you say, <name>? You up for it? I can guarantee a nice reward for you if you can bring down those shields and destroy the crystals.
It's the least we can do to keep the Scourge from making more of their own kind.
The player needs to make the killing blow in order to get credit, so pets should be left on passive while you destroy the Scourging Crystal. As the crystals only need one or two hits, you don't want your pet to be making those hits.
There is no actual need to go back and see Irena for this quest, it's simply flavour text.