This quest can be solod.
Granted, it does require DPS of certain quality. As an almost dream geared MH/BT mage with a PvP spec I'm cutting it just barely. But hey, you can bring a friend, it's still easier than assembling a full group.
It's simple. Zone in, grab a Chrono-beacon. Run somewhat close to Medivh and deal with any animals that followed you. Then move closer to start the event.
Simply put, the first portal can spawn a Rift Lord. If it doesn't, just defend Medivh and hope the second one does. Regardless, once you get a portal with a Rift Lord, go there and use your Chrono-beacon next to the Rift Lord.
The dragon it spawns has immense threat generation. It lasts only 30 seconds, however. Simply burn the bastard down with all you've got. It's safe to kill it while the dragon is alive, you'll still get quest credit. (edited this part after 4 months \o/)
Afterwards, just run out and reset the instance. Do it until you've killed four. If you brought a friend you could of course stay and hope the next portal spawns a Rift Lord as well, assuming you don't need to wait for cooldowns to produce the DPS necessary.