My FC put together a full party and got from 1-15 in a couple of hours just grinding open world mobs while doing the occasional FATE. The first 10 levels went really fast but after that it felt like it started slowing down.

We started at the eastern gate from Gridania, started killing things and working our way south to Bentbranch and kept killing things around there and the entrance to TTD while starting to do more FATEs. We eventually moved over to Nine Ivies to do levels 12-15.