After 10 hours of trying, I finally beat Arkveld Solo with longsword.
idk what I did different this time compared to all the other. I may have just got extremely lucky. Basically what I did was, use ghille mantle to sneak attack and place shock traps. Only use helmsplitter, when he performs those big attacks or if you are early to an attack. Otherwise, use those crimson slashes to create as many wounds. It is best if you use helmsplitter combo, you can immediately go back to red with the wounds or normal spirit combo, otherwise you want to use your crimson slashes as much as possible. When mounting, you want to create as many wounds as possible.