This achievement requires you to complete any 10 guild challenges from any of the 4 game factions (Assassins, Mercenaries, Courtesans, Thieves). These challenges are listed on the boards in the faction compounds (unlocked as you play through Sequence 3) and in your DNA (start menu, DNA, Guild Challenges).
These are the challenges I found the easiest:
Assassin Faction: 20 Assassin Recruits signaled to kill a target.Once you recruit some assassin recruits, just press LB near any guard. This ability has a short cooldown, so combine with other challenges for speed.
Mercenaries Faction: 10 Mercenary groups sent to attack guards.This is the classic hire/direct faction action (press Y near a group to hire, press Y near a guard to have them attack). The plus with Mercenaries is that they usually win, so if you hang around (and maybe help out) you don't have to keep rehiring groups of Mercenaries.
Courtesans Faction: 10 Courtesan groups sent to distract guards.Same as the Mercenaries above, but a group of Courtesans can only be used once.
Courtesans Faction: 10 Horses belonging to guards commandeered without killing the rider.Travel to the Antico region of Rome and run around hijacking (press B) guards on horses, there are plenty. You can even hijack and get off, then let the guard get back on (horsemen archetypes will tend to do that), and then hijack again.
Courtesans Faction: 5 Dead bodies hidden in wells or haystacks.Kill a bunch of guards (or have others do it for you), pick up their bodies (press B), and just drop their bodies into the nearest haystack/haycart/well (press B again). Repeat 5 times.
Courtesans Faction: 10 Guards assassinated from behind.Works best with small groups of guards or guards on rooftop. Just run/walk up behind them and press X. Has to be a backstab, if they turn around it doesn't count.
Thief Faction: 10 Thief groups sent to lure guards.Same as Courtesan distract, Mercenary attack, etc.
Thief Faction: 1 Sprint uninterrupted for 300 meters.Go to the Antico region of Rome, hold down RT and A and press a direction, and just don't run into anything. Comes fairly quickly.
Thief Faction: 40 Leaps of faith.Find a leap of faith (look for pigeons on the edge of a roof/wall) and jump into the hay. Repeat 40 times. Works best when you find a leap that has a ladder back to the top: There's one next to the water in the Centro district of Rome, south of Rosa in Fiore; another next to the Mercante D'Arte south of the Pantheon; a third at the Thieves' Guild in the southwest corner (credit to DesperadoRobert).
Thief Faction: Perform a dive from a height of 25 metersClimb to the top of the Tiber Island Hideout -- you'll see a triangular viewpoint-style platform to jump from, but with water instead of hay to jump into. Jump toward the water (RT + A), then immediately press A to dive.
Thief Faction: 250 f (Florins) pickpocketedWalk around a wealthy area (such as the Centro district near the bridge to Castel Sant'Angelo) and just walk around holding down the A button. You'll bump into people and take a small amount of money. Comes very quickly.
Alternatively, you can get the above challenge immediately when you earn this DLC achievement:
There are also others that may suit your gameplay style -- 5 assassinations from blend spots, 5 kills with the victim's own weapon, 5 guards air assassinated from a beam, 25 guards killed with a ranged weapon. For me, those were a bit too reliant on specific circumstances; all 10 on my list can be done in most locations and at any time.
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