I was in a similar situation, however I recently got the four Horsemen and have completed the collection of exotic weapons although it took a long time grinding raids to get it. With regards the Necrochasm, I actually deleted two husk earlier in the game and had to grind to get another. My advice is to use different weapons to kill the knight e.g. sniper, rocket launcher etc. When I did that it dropped about three more Husk of the Pit. My issue now is that I have not been able to get the Queens Cipher to drop although I have the Elder Cipher on all characters and have done POE 32.34.45 may times without success. Best of luck.