
Originally Posted by
IceSylph
Depends. If you plan on playing on Bringer of Chaos or doing superbosses, and also don't mind the main story being extremely easy (even with lowering your level all the way down), then do NG+ as it unlocks a few more Blades.
If you care about difficulty at all, then a NG file. My NG+ file is my 100% save, but my other files are all started from scratch.
Kos-Mos isn't part of the pity groups. Pity only works for 3 blades on each playthrough. It guarantees you a rare blade at 100 "points" 3 times. She has a base chance of 0.1%, but with Luck stat and using Justice Booster, it reaches to about 0.8%, which isn't that bad. I wrote a method in my guide for getting ~150-200 Legendary Cores/hour. It takes about 2-3hrs on average to get all Blades, including Kos-Mos.
Legendary Core Crystal Farming section. There's also videos online that are similar strategy, if you look on youtube.