The actual description for the achievement is as follows:
As Ullr land 20 Hail Of Arrows in a single match.
Ullr is a two-stance god which means you will need to be in his proper stance in order to activate the Hail of Arrows ability. You will know you have the correct stance when Ullr is using his bow instead of the axes.
Now that you have the correct stance, Ullr's
(ability 3) will activate Hail of Arrows. It is a small target attack with a great deal of range. You can actually stack it on multiple gods to cut down on the number of actual shots you do.
For instance; If I successfully hit 3 enemy gods with a single Hail of Arrows it will count for each god as a hit.