After recently speaking to a mate,.... searching Google..then trying the method below (for 1 level to test the method),,,I can say it works if you want to get 3 Onyx Stars
You still have to play through every chapter & level to try to get 3 stars tho.
You will need at least 1 other player to help you with this (unless you play games like me..and play them through legit on insane level all the way through with mates if need be)...
The idea here is... one player will play on on insane and the other will be on casual.
There is a link here to give you the info
or you can continue to read what I tried for a mission with a mate here...........................
The person who wants to go for the 3 Onyx stars on Insane (1st player) sets up a "Create Public" Campaign on Insane match. (From either the Chapter they want..or from the beginning of the game)
(NOTE: to test this method I'm about to give you I tried with a mate from the 1st Chapter 1st level.. 1-1)
(1st player) After setting up their game settings...("Create Public Campaign on Insane) they wait for the 2nd player to do the following:
The 2nd player Must set up their own "Create Public" Campaign match on Casual . Then the 2nd player continues to play part of that round for a short while (preferable start from the beginning of the game 1-1)..then after a few seconds or minute of playing..... Press pause and "Return to Main Menu"
At this point the 1st person has already set up the match and is waiting in their lobby for the 2nd player.
(NOTE:the reason the 2nd player had to set up their own game 1st was to make sure they were on Casual..before joining their friend)
Now that the 2nd player is back to the Main Menu.. they do nothing... but wait for their friend (1st player) to send them a game invite.
Once they are both in the 1st players Lobby..they can Start the MATCH/GAME..(NOTE: the 2nd player who put their game previously on Casual...will still be on casual and the 1st player will be on insane)
The 2nd person (whos on casual now) will need to do the majority of the killing and keep the 1st player (who's on insane) out of the firing line as much as possible as the 1st player (on insane) will die more quickly.
Continue to play the declassified like this as well. ..
Once (or "if") you have got 3 stars before clicking the Up on the D-Pad to continue/end the mission,... The 2nd Player (on casual) MUST LEAVE BEFORE THE STARS GET COUNTED AT THE END OF MISSION.. otherwise the 1st person WON'T get the 3 ONYX STARS..
So the 2nd player returns to Main Menu..
The 1st Person can now Continue the Mission and get the ONYX STARS.
The 2nd person can rejoin them in the Lobby again once the level/stars have been finished/collectedRinse and repeat if need be.
(NOTE: I'm not sure if it will work for the next mission/level if the 1st person continues to play on into the next mission and the 2nd player jumps in to the game instead of going back to the lobby every level ..as I haven't tested it that way)
If you haven't got 3 stars by the time the 1st player leaves/before end of level...then you will have to play that mission through again together until you get 3 stars...then they (2nd player) leaves at the end of each and every level. That means if you want to do it this way..it is tedious for the 2nd player because they have to quit before the end of each level..and there are about 48 levels. ..and from my 1 level play-through ..my mate got his 3 Onyx once I backed out..but I got nothing new..that I noticed. I'm assuming that might be because the 2nd player backs out before the stars go towards the count..so this way doesn't really benefit the 2nd player from what I could tell. Someone who plays more missions this way might be able to tell me?!
I'm not sure if this way is more likely to reset any Stats or Ranks along the way or while doing it..if ther are any glitches or whatever..that's if there are any current reset issues happening with GOWJ,.so please try this at your own risk. I cannot guarantee anything..just sharing the info with you.