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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Note that in step 5 you only have to kill your specific knight, not all three.