Destroy 10 enemies with hammerOnce you’ve accumulated 400 credits or more (around the conclusion of Wave 5), during the intermission, press to access the store. On the first tab, move down a couple of spaces and select ‘Hammer I’ (the icon that looks like a sledgehammer) for either module. Press to purchase it (if this is your first purchase, you will unlock First purchase (200G)), then press again to equip it to one of your primary weapon spaces. Press to start the wave.
Using the hammer ( and/or ), just destroy 10 enemies. Some might take multiple hits, but this should unlock within a couple of waves if you are using it exclusively. Note, for the Shooter II enemy (large and rectangular), you may want to use your MG to weaken it before finishing it off with the hammer.