What is a Dark Phantom weapon?
Let's see, after you combine your Swords you'll have three slots more, one will be taken by Soul weapon. So two slots. Medusa at the end of this year will get you one great weapon and one decent weapon, so you can get 1-2 more Guardian Guns. After that you need to start limit breaking them for better stats. The problem is, we're about to get Wind Guardian, and once that happens people won't have much interest in Dark Guardian. So good luck to you.