Comment by Prinzinc on Gorilla

I don't remember but it was another Korean manhwa.  The story isn't exact but all of the major points are essentially the same.

1) Assassin class hits a wall.
2) Thrown out of the party/abandoned.
3) 40ish years old.
4) Faced impossible boss battle and "won".
5) Regressed into the past to start over.

There was even a scene where there was another assassin and the MC was teaching him how to operate his class which pis*** the assassin off.  I wish I remembered the title now.  It must have had at least 30 or more chapters.