- Estimated achievement difficulty: 3/10 [Achievement Difficulty Rating]
- Offline or Online, Solo or Co-Op: 29 
- Offline or Online, Co-Op Only: 1 
- Approximate amount of time to 1000: 20-25 Hours [Estimated Time to 100%]
- Minimum number of playthroughs needed: 2
- Missable achievements: Hmmmmm... (20G)
- Does difficulty affect achievements: No
- Unobtainable/glitched achievements: No
Step 1: 100% Zer0 Playthrough + Co-Op Achievement + TVHM
While you could technically do this with any character you like, you'd be spending a decent amount of extra time if you choose one of the other characters. Each character has a class skill achievement, and Zer0's requires that he get to Level 15. The next closest is Sal at Level 10, while the other four characters only need to get to Level 2. So doing your main run with anyone else, you'll still be required to get Zer0 to Level 15, which would essentially be a waste of time. That said, do whatever you'd like.
For this step, you'll be completing the main story portion of the game (which is just a recreation of the Borderland 2 DLC), as well as all side-quests, the arena, and the raid boss. Once all that is done, you may have a little grinding to do to finish off all the challenges, but I'd suggest taking a look at those and working on them as you play through all the other content - especially the "Open Wide" challenge for the shotgun, which you have to get to Level 5. Try to switch guns often after that to ensure you're getting kills with different types of guns, different elements, etc. This will lower the amount of grinding later on.
You'll also want to do the co-op achievement on this character, as there are three challenges that require co-op as well. Get the achievement for reviving someone on your friends list, and do that four more times to satisfy the challenge, then win a duel and trade with them as well. Can be done in split-screen with a second controller or online with a friend.
Once all that is done, you'll likely still need to grind a ton of levels to hit the level cap of 35. Play through the story again on True Vault Hunter Mode, then repeat the arena until you're done.
Warning: Be aware of the missable achievement mentioned in the Road Map. While you can still get this if you join someone else's game or find a partner who can give you the item required for it, you can miss it in your own playthrough.
Step 2: Partial Sal Playthrough
Sal is the only other character that needs to gain a decent amount of levels to perform his skill achievement, requiring Level 10 and two specific skills to unlock it. Play through the content again and/or go on killing sprees to get the required level and nab his achievement.
Step 3: Other Four Character Achievements
The final four characters in the game simply need to gain one level and be Level 2 to unlock their special skill, then use it 100 times in the case of Maya or kill 100 enemies with it for the other three. As you level up a bit from these kills you may be able to grab skills that will increase your special's effectiveness, but it shouldn't be necessary. A good spot to get all these is in Flamerock where the Lootapult lands, a group of 7-10 spiders will spawn. Save and exit after killing all of them to have them respawn when you load back in.