Right now we're still waiting for areas to release, so everything past thunder is difficult to come by. Gacha and raid bosses are the only ways to acquire decent water/fire/light/dark weapons and eidolons. My best weapon is an SSR dark gun, but the whole gunslinger tree is such crippling overspecialization that I wouldn't spend into it unless I had SP to burn.

The first event in the dmm version (Apocalypse) was dark based, so it's probably safe to work on light first.