You can play in Survival ll and stil get this achievement, that way you wont need a third playthrough.
Playing in NG+ makes the game way easier, since you have all the credits from your first playthrough.
The horde freakers always have the same health (unlike regular freakers, whose health increases according with the difficulty), so dont be worried about fighting a horde of sponges. Plus, only 2 hordes are necessary to take down in order to complete the game, the other ones being optional.
Isnt really hard, its just more time-consuming.