Getting the exotic swords, but I've heard they're such a grind. Just haven't had enough interest.
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I did it for the solar and I'm glad I did but I don't know if ill ever do it for a second time.
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I just completed it for my Arc sword. It actually wasn't bad. The material u need is spin metal which in my opinion is the easiest to farm on earth and I only had to get 100 pieces. (Every guide I read said 150). All in all, I just chipped away and did a mission towards it once a week or so.