Croman
1. Kill all mobs on the way to the Magmolatus boss, but leave the boss alone.
2. Kill all mobs on the way to Slave Watcher Crushto and kill him; talk to Croman.
3. Kill Magmoaltus and let Croman get his sword.
4. Make your way to Roltall and kill him; keep Croman healed at all times.
5. Make your way to Gug'rokk and kill him, wait for Croman to do his thing and get the achieve announcement.

AknorSteelbringer
Defeat Flamebender Ka'graz in Blackrock Foundry without killing Aknor Steelbringer on Normal, Heroic or Mythic .
Flamebender Ka'graz's special achievement is The Steel Has Been Brought. This specific achievement rewards the epic follower Aknor Steelbringer.

Millhouse Manastorm
This follower is awarded from completing For The Children!, which require you to loot a Miniature Iron Star in the room before the final boss in Upper Blackrock Spire.
Millhouse the quest-giver periodically appears on the top floor of your Inn. To get him to respawn, move your Inn to a different plot in your Garrison by visiting the Architect's Table.
As Millhouse only spawns on the top floor of your Inn, you will need at least a Level 2 Lunarfall Inn / Frostwall Tavern to access this quest giver.

Leeroy Jenkins
For The Children!, which can only be completed in Heroic mode, rewards players with Millhouse Manastorm as a Garrison follower.
Additionally, players can complete the Leeeeeeeeeeeeeroy...? achievement in Heroic Upper Blackrock Spire to receive Leeroy Jenkins as a Garrison follower.
To complete this achievement, players must loot Leeroy's corpse in the Rookery after killing Kyrak, resurrect it, and then killed all mobs and bosses up to and including The Beast before Leeroy's 900-second (15-minute) "chicken timer" expires. If you do not clear all of those mobs, he will run around pulling everything in the area and die.

Meatball
Boss Sequence to get Meatball: Rank 1: Goredome, Sanoriak, Smash Hoofstomp, Akama Rank 2: Dippy, Kirrawk, King Kulaka, Fran &Riddoh Rank 3: Blat, Vian the Volatile, Ixx, Mazhareen Rank 4: Tyson Sanders, Nibbleh, Dominika the Illusionist, Meatball