Although the quest log clearly states that you need to ride a griffin, and even tells you who to talk to :) I honestly didn't read it either and was killed by an elite while I wasn't paying attention before I thought "wait a minute...I can't kill those guys at this level!" and went back to see what I was missing. Wish I had done so sooner...the guy that loans you a griffin is RIGHT NEXT to the guy that gives you a quest. How simple can it get? ;) Just note that he won't have a "?" or anything...you just talk to him and click the little text bubble and he throws you on a mount. It does all the driving, you just aim and throw when you get to the shrine-looking thing that's being guarded by a bunch of elites (look for the big green crystals above them..you should have already blown up some similar ones in a previous test, just with weaker guards).
By the way, the guy you talk to for the griffin ride is at about 71,62 and the questgiver is right next to him by the campfire. The two gateways are at about 78,47 and 78,51 (just outside the dark portal). As several people have already said, make sure you do this before the bombs time out! lol