The xp required to get one friend to a level 5 ally is obscene, but then to have an achievement tied to doing this with 4 other people is ridiculous.
The fastest way to do it, which is grinding against AI (which is boring) takes over 80 hours. Doing it on something else like horde or actual versus could take hundreds of hours.
This should really be addressed. At present, for a lot of people, the achievement for playing with a full squad of level 5 allies is more time consuming than the seriously achievement.