Wingman

To get a follower to be your 'wingman' or bodyguard, you need a lvl 2 garrison, lvl 2 barracks, and at least one of the 5 followers that can be bodyguards. Finishing the introductory quests in Ashran gets you one (Alliance: [faction=1733] , Horde: [faction=1739] ) so that is not too bad.

To get the bodyguard to accompany you, select the barracks in the upgrade/architect window. Select 'assign follower' like you would if you put a follower to work in your sweatsh- profession buildings. Select the follower you'd like to have as a bodyguard. He/she/it will be spawned next to your barracks, and you talk to them to get them to follow you.

Once they are following you, you can kill any creature to get reputation with them (critters and some other popular exploits were hotfixed, so its mostly normal mobs that give rep now). You get 10 rep per kill (11 for humans). There are three rep levels (body guard- trusted bodyguard - Personal wingman), requiring 10000 reputation each. If they die, you get a 1hr debuff during which they are unavailable (but you can change the assigned follower and take another with you) and you can recover them after this in the same location you talked to them before.

At 110 you must either go to tanaan, or to any of the area's that offer max-lvl dailies in dreanor. Lower lvls can still do many of the methods listed in the posts below. At 120, it was very difficult to find mobs that still count, but it appears to have been hotfixed so max lvls can still get this achievement.

Ishaal is the best "wingman", right?

Wingman FARMSPOTS:
There are currently 2 working farmspots where you can easy get the 20.000 reputation in 1 hour.

1 Spires of Arak: Kill [npc=84905] at 58.6, 45,0 Map: http://www.wowhead.com/maps?data=6722:584458
Pro:
+ Fast respawn it needs 2min 10sec for 1000rep = 43 min to max 1 follower
+ Follower almost can't die here exapt if you let him solo the rare mob.

Contra:
- can't go brain AFK here as soon as you have killed all the small ones it triggers the Rare mob [npc=84912] also the level 96 mobs to realy hard dmg if you pull them all.
- no loot or Items

2. Talador Kill [npc=xxx] at 56.4, 63.4 Map: http://www.wowhead.com/maps?data=6662:564634
Pro:
+ Very fast respawn it needs Xmin Xsec for 1000rep = 45 min to max 1 follower

Contra:
- Follower can die here very easy! Make shure you are a guardian Druide or any other Tank. As dps you will have to be very carefull to not let your Follower or yourself stand in one of the 3 ground fire effects!
- no loot or Items

3. [zone=6719] Kill [npc=82371] at 55.2, 74.6 Map: http://www.wowhead.com/maps?data=6719:553747
- Respawn is Slow (patched)

4. Gorgrond: Kill [npc=83523] at 52.3, 71.2 / 52.5 70.0 / 50.9, 71.2 / 51.2, 72.9 / See Map of: [npc=83523]
- Respawn is Slow (patched), you have to run around here...