Maybe some assasins should have a 1600 ms one hit kill like old shugoki DE. I know that might sound overpowered but they are assasins after all. I agree that assasins like Pk have it rough and they aren't a threat in most cases. In 4v4 they are a burden on their team especially when they rage quit.
If we create a scenario where they can stealthily deal massive damage to an opponent I believe the game would be more interesting.
If the shot missed it gives massive revenge (like a parry) and damages the assassin or leaves them vulnerable like old shugoki.
They can only use it at critical health and if they use it while not critical it does 40 damage if it misses and restores 80 if it lands.
The shot can be blocked, parried, or dodged but has HA 600 ms in.
It could just be added to forward lunge attack.
The attack while in critical would do 2000 points damage if it lands.
The assasin can not die from receiving damage by the use of the attack meaning if they have low hit points they will always have 1hp left no matter how many times they try it. So missing while at critical will only damage the assasin down to 1 HP and no more.
The attack can be done off a wall splat and it's guaranteed. It also guarantees an execution.

This would make a wounded assassin a real threat if backed in a corner or being stealthy and more fun to play imo. It would also create better synergy between nonassasins and assasin combos.

An assassin is about finding that perfect time and place to attack. This type of mechanic would personify what it means to be an assassin. It's exactly opposite to light spam.