You can get by the three doors to get to Fenrir with someone who has the portal charm (one needed for group going through, whether a party or not), or with a WHM, BLM, and RDM who all stand on the appropriate square, or by completing the series of quests that takes you into the tunnels from Windurst Walls (the one with the Star Sibyl's tree). No rank requirements required for any of that unless you're the one who wants to get the portal charm.
If you've beat a level 20 avatar, the fork will no longer transport you out to the fight.
If you've gotten the avatar from the level 65 avatar, then the fork can still be used to warp up IF you warp up as SMN main.
Getting to Fenrir should never be the problem - fighting him is a bigger hassel.
Leviathan - PLD90, BLU90, MNK 90, DRG 78