As stated by Spice, Do the Disaster Raids (Standard and Expert Difficulty) or the Catastrophe (Ragnarok Difficulty). If you do the latter one, I suggest doing it by joining someone else and leeching it (preferrably when it's close to dead and not when a lot of ppl are there, because you don't want to doom it to failure). The Ragnarok only drops the Atk skill SR weapon (when it does drop) and the former 2 difficulties have a chance to drop 3 SRs (2 of which are HP skill).
Next event is vs Fire so find a good union that can clear the content so that you can MLB the weapon. And then 2 events after it should be another fire event, so you'll have another chance for another Fire weapon.