Okay... Not a repost, in spite of similar title.
What you need:
310% speed flier.
Iron Boot Flask.
1. Mount up on your 310% speed flier OUTSIDE Netherwing Ledge.
2. Use the Iron Boot Flask WHILE MOUNTED.
3. Fly to Netherwing Ledge.
Normally, your 310% flyer will be replaced with a 280% Dragonmaw Netherdrake. This prevents that.
You cannot mount while affected by the IBF but you can use the IBF while already mounted. Normally when you approach NWL, you "remount" in midair with the Dragonmaw Netherdrake. However, with the IBF active, you cannot "remount" and so you are able to ride around Netherwing Ledge on your normal mount. Thus, your 310% flier is not replaced by a 280% flier.