Im an assassin main and I love set, though i never play him cuz he is broken (in a bad way).
I have come up with some solutions.

I think generally speaking his kit is fine.

his 1 and 2 need way more damage. (I was almost constantly hitting gods with the 2+1 combo and only did about 10K player damage)

His 3 is useful but it can be more so, the teleporting is cool but make that toggle to his 2 maybe and focus on the damage reduction/ damage aspect.

his uli is hot garbage. Idk how many times ive been kill right before my 4th basic because ive been hard CC'd plus the heal itself isnt all that good. maybe put some CC immunity in it or some better dive in there for more usefullness. Also maybe change an aspect on his basic attacks by adding range or some AOE while in this form.