Each time that I have won against Fenrir my party set up has been different. The first time we had whm (me), smn (70+), drk, blm, blm, nin. If you can survive up until the 2 hour at 50%, and get through the 2hour in one piece then you are almost guaranteed a win from what I've seen.
I've also fought fenrir with a leech (someone below level 70) in the party 3 times before. Two of those times it was a smn who needed fenrir and another time it was a blm (Arcanis) who was level 61 and stunned Howling Moon like a pro! /cheer Arc
Just recently I was able to get my smn to level 71 and entered a smn fenrir party. It was 5 smn and a rdm and we won in no time and it only took us 4 minutes. Of course summoner parties doing fenrir is the easiest way to get through it but not everyone has a level 70+ smn. I think as long as you have one level 70+ smn (Spinning Dive and Mountain Buster are amazing against Fenrir and pull hate off of the tank) and a very good tank and whm you can get through this just fine. A bard can help too of course if they keep dark carol up, but that is if you are taking a melee party in and Howling Moon doesn't get stunned.
I could go on for a while talking about my strategies for Fenrir but I won't bore you guys. If you need a 71 smn to help you then count me in. I love fighting him! Send me a tell any time and the same goes for anyone else reading this post who needs a smn or a whm for fenrir.