I don't see any point in getting the Exotic sword with the ridiculous amount of work needed to get it.
Step 1: Get the materials for legendary sword
Step 2: Use sword against 50 majors/ultras, and 25 guardians
Step 3: Farm a "-blam!-" ton of the material the sword was made from for "Core" pieces and get ability kills to 100%
Step 4: Wait till Arms Day
Step 5: Kill Alak-Hul and 3 knights within 30 seconds as if a nightfall from year 1
I just do not think this amount of effort is worth it. I believe you should just have to finish the strike to get the sword as it is damn near impossible to get this done without extreme luck.
This is just my opinion, and some may agree and some my not. But before all you tryhards and "MLG" (Cough), post about me being a wimp, loser, etc. This is my opinion for something I believe is not worth the effort and should be balanced out.
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To clarify, you don't HAVE to kill all three knights, just the one that is required for your sword. It will take multiple runs. Is it easy, no. Is it doable, yes. My advice, get a team together that you can communicate with and understands what is going on. Most randoms appear to be void of any intelligence that is necessary to function. As for the collection of the materials, it was INCREDIBLY tedious but if you want something bad enough, most tasks will seem small to aquire it.