RWBY: Grimm Eclipse

*Still working even after the latest patch*

This solution is the "cheap" way to unlock the achievement without even bothering on resetting all your character progress.

I didn't think it would work but for my surprise it does. It's the "delete the gamesave from the console and restart the game again" method that some games like Diablo 3 have.

So pretty much what you are going to do is level up all your characters to lvl 10. As soon as you do, close the game and reopen it (this is so the gamesave gets synced in the cloud)

Now what are you going to repeat 10 times is this:

-Go to "Rank" in the main menu, choose "Go to next rank". This will re-up you to the next level.
-Press and then (This part is important, DON'T QUIT THE GAME)
-Choose Manage Game
-Go to Save Data and choose your gamertag's gamesave
-Press and choose DELETE FROM CONSOLE (Don't shoot yourself in the foot and delete it from the console and the cloud by choosing the other option)
-Press or and load the game again.

As you will notice, the option to rank up is available again, so you are going to repeat these steps 10 times until the achievement unlocks. This is for people who don't want to be constantly deleting their characters progress every time to do it the legit way. Hope it helps you people out.

*NOTE* As Ghost m0de noticed, you might need to do the DELETE FROM CONSOLE step twice to completely erase the gamesave. I just needed to do it once though
*NOTE 2* LAZY SHIKAMARUX confirmed that if you have a more advanced rank than the 1st one, the method can still apply when you meet that specific rank's requirements

I recommend everyone take a peek at the achievement guide posted by Michel on this as this saves a lot of time.

However, im going to leave this up in the event that this method get's patched somehow and it needs to be unlocked legitimately afterwards, or for those who would like to do it in these steps. FYI I would not recommend doing it this way as this adds approximately 40+ hours to your achievement hunting, if not much more, but that being said, do as you wish! :)

QUICK NOTE: If you do screw up his guide, and have to do Rank 2, you LOSE ALL LEVELS meaning that you have to replay the game to unlock chapters again. Also, there's a couple people that have Ranked Up via glitch or legitimately only to log in and be Rank 1 Again, do the glitch, do it in one sitting, and seems if you play solo there's no issues but online modes tend to occassionally and randomly reset progress, levels on characters etc. If you accomplish to beat the game, level up all the characters and whatnot offline or do some online and it saves progress and do a single player level, you should be fine.

Thanks to the Developper for this! RoosterTeeth Games
This is their guide, I take no credit minus anything under NOTE at the bottom:

HOW TO ADVANCE IN RANK?
To help with any confusion on Ranking up. Here's a quick guide for your reference.

RANK 0 -> 1
All players begin at level 0. To achieve rank 1, you must meet the “Maxed Out” requirement -
Maxed Out: Reach Max Level (10) with four characters

RANK 1 -> 2
Rank 1 players will be promoted to rank 2 after completing the following challenges:
Leveled Up - Get 1 character to level 10
10x Revives - Revive 10 teammates
50x Ranged Kills - Get 50 kills with ranged attacks
50x Hit Streak - Get a hit streak of 50 or higher
Ursa Slayer - Kill 20 Ursas

RANK 2 -> 3
Leveled Up - Get 1 character to level 10
Creep Killer - Kill 300 Creeps
20x Team Attack Finishers - Perform 20 Team Attacks
50x Counters - Perform 50 Counters
4x Challenge Master - Unlock 4 Mastery Challenges

RANK 3 -> 4
Leveled Up - Get 1 characters to level 10
Beowolf Buster - Kill 300 Beowolves
20x Team Attack Set Ups - Set up 20 Team Attacks
100x Destructibles - Break 100 crates or pots
10x Artifact Collector - Collect 10 artifacts

RANK 4 -> 5
Leveled Up - Get 1 characters to level 10
Alpha Creep Killer - Kill 150 Alpha Creeps
5x Flawless Waves - Get 5 Flawless Waves
20x Revives - Revive 20 teammates
100x Ultimates - Perform 100 Ultimate Attacks

RANK 5 -> 6
2x Leveled Up - Get 2 characters to level 10
Alpha Beowolf Buster - Kill 150 Alpha Beowolves
6x Challenge Master - Unlock 6 Mastery Challenges
10x Wave MVPs - Get 10 Wave MVPs
15x Artifact Collector - Collect 15 artifacts

RANK 6 -> 7
2x Leveled Up - Get 2 characters to level 10
Boarbatusk Butcher - Kill 100 Boarbatusks
100x Stuns - Stun 100 Grimm
100x Team Attack Finishers - Perform 100 Team Attacks
50x Assists - Perform 50 assists

RANK 7-> 8
2x Leveled Up - Get 2 characters to level 10
30x Ursa Slayer - Kill 30 Ursas
Mutant Beowolf Buster - Kill 20 Mutant Beowolves
Android Annihilator - Kill 200 Androids
8x Challenge Master - Unlock 8 Mastery Challenges

RANK 8 -> 9
2x Leveled Up - Get 2 characters to level 10
Mutant Creep Killer - Kill 100 Mutant Creeps
200x Stuns - Stun 200 Grimm
75x Hit Streak - Get a 75x hit streak
50x Wave MVPS - Get wave MVP 50 times

RANK 9 -> 10
Maxed Out - Get 4 characters to level 10
Grimm Reaper - Kill 3000 Grimm
100x Hit Streak - Get a 100x hit streak
10x Challenge Master - Unlock 10 Mastery Challenges
50x Flawless Waves - Get 50 Flawless Waves


NOTE:

- Anytime you go up a rank, any kills, streaks, revives etc MUST BE DONE AT THAT POINT, no previous progress is recorded.

- At the beginning of Level 10 - Final Exam, you can turn immediately to the left and pick up a collectible from spawn. You can run forward, die, restart the level and pick it up again, this grants you 100xp everytime you do it which saves, making a quick level grind. You can also EXIT and it will save your XP, but that being said it's quicker to run in and hit the exploding beowulf creatures and die.

- One of the quickest farming methods ive found is in 2 player co-op in the mountain moving the nuke with fuel, have one person fill it while the other fights and milk out the infinite spawns till the last second.

- For URSA FARMING, I used level 5, on Eclipse difficulty which only adds more mobs not make them stronger or harder. If you go through the Right Door (same one for the Clever Platforming Puzzle Achievement) one of 2 waves will spawn, either a bunch of the regular Grimms, or 2 Ursa followed by 4 more. These spawns are random which it triggers (the groups) so just keep exiting and doing this to get roughly 6 kills very quickly. I also recommend Weiss at Level 10 with her 2nd Ranged upgrade ONLY which does a ton of damage. Rank 3 freezes them and removes the damage buff so do not level this past 2.

Each character fully levels to Level 10, this does not take too long to accomplish (in a Single Player run looking for the collectibles I hit this naturally)

Level 1: 500 xp
Level 2: 750 xp
Level 3: 1000 xp
Level 4: 1250 xp
Level 5: 1500 xp
Level 6: 1750 xp
Level 7: 2000 xp
Level 8: 2500 xp
Level 9: 3000 xp
Level 10: 4000 xp

Total XP Required (1 Character): 18,250

Rank 1 Total XP Required: 73,000 XP
Rank 2 Total XP Required: 18,250 XP
Rank 3 Total XP Required: 18,250 XP
Rank 4 Total XP Required: 18,250 XP
Rank 5 Total XP Required: 18,250 XP
Rank 6 Total XP Required: 36,500 XP
Rank 7 Total XP Required: 36,500 XP
Rank 8 Total XP Required: 36,500 XP
Rank 9 Total XP Required: 36,500 XP
Rank 10 Total XP Required: 73,000 XP
Grand Total XP Required: 365,000 XP