How does that play out?

For example a player hits a NSC, who parries. So the player got the requirements for assassin strike, the NPC got the requirements for a counter attack. This leaves the following possibilities:

A: Player uses his assassin strike first, NPC can't use counter attack as he's out of range

B. NPC uses counter attack first, now it gets complicated:

B1: Player uses dodge or parry, so his last action wasn't an attack. Is he allowed to still use his assassin strike after this?

B2: Player takes the hit. His last action was an attack, so he should be allowed to use his assassin strike now.

How would you handle this? Comparative rolls (WS? AG?) to see who comes first? And how would you handle the whole B situation?

The wording of the talents suggests to me that Counter-strike should take precedence. It is "immediately make" instead of "after making a melee attack" as Assassin Strike.

And continuing from this, technically Assassin Strike happens "after making a melee attack".

So my conclusion is:

1. PC attacks NPC.

2. NPC parries.

3. NPC counter-attacks.

4. PC gets to attempt dodge or parry.

5. PC gets to use assassin strike.

As long as still have reactions left to dodge and/or parry

Tecnically you cannot use a reaction on your turn. There are exceptions to this rule, but in this case I have run the game as follows:

1. PC attacks NPC.

2. NPC parries.

3. NPC counter-attacks.

4. PC gets to use assassin strike.