Get to Work!- As Fafnir boost all of your allies Attack Speed in a single match using Coerce.
This is easiest done on Joust. However since it's one of your beginning skills and not your ultimate. It's still easy to get in arena. Plus the other achievement for Fafnir is easiest done in Arena so you might as well play arena. You have to use the Coerce ability and use it on everybody on your team.