Curiosity got the best of me so I decided to find out how long it would take me to unlock the Deebee, Cog Gear, and Warden(I already had RAAM from the Gears 4 Gilded challenge) I did everything exactly the same except for each characters unique challenges, all in all it took me about 18 hours over 3 days of continuously grinding what I believe to be the most efficient method to complete each challenge.

50 waves of Horde: I joined public Terminator playlist on Insane(I didn't care if we finished or not, because I also wanted to maximize XP) This also allowed me to grind the new medals and complete that section for the banner.

18 chapters of escape: Custom Beginner Escape on The Surge as Mac. Run straight to the end, taking the left path at the beginning and right path after the checkpoint. As long as you have your bleed card the final wave of enemies can all be 1 or 2 tapped and will bleed out before they reach the door. 9 speedruns and you're done.

40 rounds of versus: Co-Op vs. AI and always vote Dodgeball. Very simple.

60k XP: I always had to finish this one last, and I usually just went back into Terminator Horde on Insane for reasons stated above. As long as you work on the rest of the challenges, this one won't be too bad to finish up. 20 waves or so.

Now for the unique challenges.

Cog Gear 200 Assists: As you're working on the other challenges, try to always use your lancer to mark and fire a few bullets into all enemies, allowing your teammates to snag the kills. Very easy to do in Horde and Co-Op vs. AI.

Warden Kill 10 Wardens: Custom Beginner Horde on The Gauntlet. Again I use Mac for the OP bleed damage with the boltok. You'll have to kill 2 Scions in the 1st room, then either another Scion and Stump or 2 more Scions, then the same in the last area, with Juvies, and Imagos or Hunters sprinkled in between. There's usually an ample supply of Salvos and Dropshots to make it easier, after you clear the 3rd room you will always be ambushed by Juvies and a Warden, either a few Salvo rounds or Dropshots make quick work of him. After you kill him, die and restart the chapter and do it all over again. VERY IMPORTANT NOTE: You must finish the match after you've done this 10 times or it will not count. As long as you finish, it will count your Warden kills no matter how many times you decide to restart in one session. Just remember to finish the match when you're tired of grinding.

Deebee kill 100 rejects: Same type of grind as above, Custom Beginner on The Hive. There is some RNG involved here, but most of the time you will get at least 10 Rejects in the first area. Kill them all, die, and restart chapter just like with the Warden. VERY IMPORTANT NOTE: You must finish the game after you're done grinding for it to count your progress. So if you restart 4 times and kill 40 rejects, complete the game on the last attempt or it will not count any of those kills.

So that's what I did, I hope this helps some of you speed up your grind. I can honestly say outside of the Terminator Horde, none of this was fun, but at least I'm done and I didn't pay a dime. If anybody else has any tips, tricks, or alternative methods that you believe are faster please share them here. Take care and good luck!