Gears of War: Judgment

If you are extremely good at Gears, you shouldn't have too much trouble, but here is the easier way of getting this achievement AND completing the game on Insane AND Onyx stars!

- Switch Party Type to Local and sign in a guest on a second controller.

- Set your difficulty to Insane and the guests difficulty to Casual.

- Play each declassified mission as the guest on Casual.

*This method won't get you Onyx Stars*

Thanks to HolyHa1fDead, there is a way to get Onyx Stars too using this slightly different method:

- Have a buddy join the game and he or she plays on casual.

- The casual player quits before the end of the mission, then you will get the Onyx stars.

*The Casual player can not get the Onyx stars by rejoining (as you don't get to choose your difficulty for an already occurring game.)*
3 options:
1) Take turns on each mission being Insane player.
2) Play entire Campaign as Casual player and then swap.
3) Have a buddy who doesn't care about achievements but doesn't mind helping you out :D

Here is a link to a video walkthrough on insane, getting all the stars, and every declassified mission. It has full commentary and should help players of any skill level get these achievements.

http://www.youtube.com/playlist?list=PLTaOHX8kEkD77HyFcClXIm...

Hope it helps.

This can be done solo as I just did it. Only a few levels get really hairy. When it doubt, just RUN. RUN AWAY. Run past your team and let the toughest enemies (i.e. Ragers, Bloodmounts) attack your team.
But let's all get real. This achievement is about beating Karn on insane-declassified. It is virtually one of the hardest levels you will ever play in video gaming. It takes the perfect mix of prep, luck, and patience. Here's my guide after over 100 attempts and then beating him twice in 4 attempts.

Round 1 Karn's Guns:

You should never die here before Karn gets up on the roof the first time or you are just rushing. To prepare, work your way over to the right, and gather up full lancer ammo and the boomshot and grab the 2 frags. Perfect hiding spot is in cover, in the front of the circle where you can see where they were spawn from.

Round 2 - 7 Torques, 1 Mauler, and 4 Cyclops - When the 1st round of elite spawns, wait a few seconds for them to congregate near your team in the middle, and then quickly spam as many of them with grenades and boomshots as you can. Avoid the mauler at all costs unless you can get a grenade tagged on him. This is the most vital part of the level and where luck plays the biggest role. If you don't get enough guys with grenades and boomshots, you might as well restart bc a torque bow will most likely get you. In the meantime, STAY RUNNING from Karn's charges. Hide behind the taller of the covers when his leg is exposed. This keeps you out of the eye of torque bows. Dont use cover though, just move as close to it and active lancer the piss out of his leg. Two or three clips should do it.

Round 3 - Run away from nemacysts and make your way to the markza and pick up the other 2 frags. Hide in the middle area where the frags are until the next spawn comes.
7 Ragers, 2 Grinders - Markza the piss out of the Grinders. They like to end up right where you need to hide. This part is as important as dealing with the torque bows - ACTIVE MARKZA THE RAGERS. They make for easy head shots and 6 or 7 actives will take them down when raged. Meanwhile, with your active lancer prepped, keep a lazy eye on Karn's mouth. It doesn't take much damage at all and if you can down him with no grubs left, you are GOLD. GET TO COVER AND BLIND FIRE LANCER. His pistols will sneak up on you as the red is somewhat hidden over the pet dog that he is trying to recover on. Spam him and then cutscene, jump out of your chair, kiss your dog, go buy some taco bell and go brag to anyone you can that you just beat one of the hardest levels of all time in video games!!!!! Good luck!