All we needG 5

All we need G5:
Beat a wave without summoning allies

Allies don't become available until level 2, and for some reason this achievement, even if you do not use allies, will not unlock on level 2 since they want you to learn how to use allies in that level as part of a tutorial. I recommend doing this on waves 3 or 4, as those are very easy to beat the zombies without the use of allies. Just simply run up to the zombies and use your auto attacks and Katana Slash on them and you can do this very easily.