It's been a while since I've been on here so sorry if these have been posted before: I posted a couple of these on Reddit too
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Ussop will be able to lie to CoO users, embracing "The liar" moniker. He could hide from observation tracking and bombard them with false red flags. Like a sniper shot from behind when it's actually a smoke bomb from his left.
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Back in loguetown they told a story about how the 21 great swords have demons in them IIRC. Asura is Zoro manifesting the haki of one the the "spirits/demons" in his swords. Later on asura and his presumably two other forms ( one per sword) will be more like fighting styles than a direct power up. Kinda like 1 sword style vs 3 sword style, or G2 and 3 currently. I can see his other forms giving him more reach and one to give him more mobility.
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Rokushiki has the best tools to combat a haki user. Soru ( and being stretchy) allowed Luffy to outblitz two of the snake sisters Observation Haki. Tekkai might be good against armament too since it makes the body tougher. Kami-E and geppou allow for some abnormal movement which might be hard to predict for haki users. However it also is a really good suppliment to Haki as well. Armament plus tekkai should be tough as hell to bypass.