Diablo IV

When playing the dlc campaign you will unlock a new area called Nahantu just follow the yellow quest marker, you can't miss it, moving right along you will come across a place called the den, when the mission is done you get your first mercenary his name Raheir, go to him talk with him and hire him, he will be your party. After that you will see purple markers on your map go to them there's 3, then go out and get the last 3 Mercenaries you need, There is only 4 Mercenaries in the game when you get them you can hire 2 at the same time so this achievement won't be that time consuming. You level them up by killing elite enemies bosses, and leveling up yourself, this will take awhile. Although since you can have 2 of them with you one will get only 50% of the exp sadly.

PC players read first!. If you are like me and you plan on playing this on PC and then doing a quick load up on XBOX for this achievement to pop your final level 10 please be aware: You must pop level 10 on each mercenary on the Xbox. The achievement back end tracks each one for 25% of the achievement. You will not unlock the achievement for simply leveling the final Mercenary to 10. I learned this the hard way :(

Rapport is shared account-wide among all your characters that exist in a particular Realm: Seasonal, Eternal, Seasonal Hardcore, and Eternal Hardcore. So if your like me and you leveled 3 of them up and tried hitting level 10 on the 4th on the Xbox on Seasonal Softcore. You can then go to Eternal Softcore and level the other 3 up to 10 on the Xbox and achievement will pop.

Now on to the guide:

As you progress through the Vessel of Hatred Campaign you will unlock Raheir, which will in turn unlock the Den:

The green diamond-shaped icon northeast of Kurast Bazaar takes you to The Den.

Once he is unlocked go up to him and "Hire as Mercenary". This will also unlock 3 other Purple diamond quests for the other 3 mercenaries, (Varyana, Aldkin, and Subo).

Quests names:
Raheir: Complete the Act VII main quest, "The Hand Remembers the Blade"
Varyana: Complete the Priority Quest, "Slayer's Retribution"
Aldkin: Complete the Priority Quest, "A Nameless Mystery"
Subo: Complete the Priority Quest, "A Feather on the Scale"

When you unlock a 2nd Mercenary go up to them and "enlist as reinforcement". I would then make sure you activate their skill on "when you use a skill in combat" though having a skill equipped is not necessary to earn rapport. Your reinforcement will earn 50% of the rapport your Main Mercenary earns.

Now you get Rapport for killing any kind of elite or completing a quest. This is what you need to level a Mercenary up. I wouldn't worry to much about this because as you play the game you will level them up fairly quickly. Just make sure as soon as one hits lvl 10 you switch them out in "The Den".

The fastest way to level these guys up is the Infernal Hordes. Always pick hellborne as they count as elites. If you get the negative effect of "Elite chance is doubled" definitely pick that.

The amount of rapport you earn per elite kill:
Torment 4: 33 Main / 16 Reinforcement
Torment 3: 28 Main / 14 Reinforcement
Torment 2: 24 Main / 12 Reinforcement
Torment 1: 19 Main / 9 Reinforcement

Hope this helps! These are the things I have learned from doing it twice since I originally did it on PC. Keep blasting!

When playing the dlc campaign you will unlock a new area called Nahantu just follow the yellow quest marker, you can't miss it, moving right along you will come across a place called the den, when the mission is done you get your first mercenary his name Raheir, go to him talk with him and hire him, he will be your party. After that you will see purple markers on your map go to them there's 3, then go out and get the last 3 Mercenaries you need, There is only 4 Mercenaries in the game when you get them you can hire 2 at the same time so this achievement won't be that time consuming. You level them up by killing elite enemies bosses, and leveling up yourself, this will take awhile. Although since you can have 2 of them with you one will get only 50% of the exp sadly.