Do you mean from scratch or if you already got the achievements before 1.5 and need to get from 5 to 7 now?
Normally the best way is to get as many achievements as you can, so just get a group and run every dungeon. Then I'd just do daily pledges. I don't think grinding a low level dungeon will help, it does count towards damage/block/heal achievements but with enemies having so little health they will take forever...
The only way to level up the Undaunted skill line is to do achievements related to dungeons and the Undaunted daily pledges. If you don't feel like grinding out all of those achievements at once I would suggest just sticking with the pledges since those should also net you a ton of experience through the skill line at a steady pace.
-Pryda - Ebonheart Pact XB1 NA
World first HelRa Hardmode player.
GT: Bootleg Mix