Hi there Sohan. Is the dark sword the only item you're missing? Or more?
The only tip I can give you is to draw item using the 10 souls draw. I got more swords using that method. By combine method I don't have any luck. You may combine your extra swords but there is no guarantee. I can say drawing from shops is faster.
I may get more playing time for now cause our region is in a lockdown.