Although Nonolato advises you to do this with a group of friends, it is not too hard to solo. I did this at Rank 36. After you meet up with Keelty at x32 y26 she tells you to run and a whole bunch of Yarzons spawn (all this is in the cutscene). You will be able to outrun them and when the cutscene ends you end up next to the Sorrel Haven aethertype. Touch it if you have not already done so. Your journal now tells you to go back to where you met her. There are two ways to do this.

(1) If you don't mind using one anima, use the Sorrel Haven aethertype to return to Bentbranch and make the long (but safe) run back to x32 y26.

(2) if you don't mind some danger and want to take a quicker way back, make sure Quicktride is equipped. The run from Sorrel Haven is filled with a bunch of nasty aggro mobs. Get as close to the first aggro mob as you can without drawing aggro, hit Quickstride and run (make sure you hit F immediately after Quickstride so that you can run faster). You'll get hit hard at least once, but by the time Quickstride wears off you will be at a fairly safe resting spot. Wait for Quickstride to refresh and again run until all the mobs return to their area. You are now perfectly safe. Go back for another CS with Keelty.

In the next CS Siward spawns and you are to defeat him. He is some distance away and surrounded by Yarzons, but everything is far enough away so that they will not aggro. Once you engage Siward, he will come towards you with all the yarzons, so you have to beat him quickly. What I did was load up Multishot, hit Raging Strike and Hawk's Eye, engaged Siward, and used a weapons skill every time I had TP. The fight was over fairly quickly and all the yarzons disappear once you beat Siward. From that point on there is no more fighting and you go back to Gridania to collect your reward.