Personally I found it best to get as much of the game's optional objectives done before going ahead with Arador's method. You unlock cheats from Abstergo challenges but since this will probably be the last achievement you'll be going for, chances are you'll have the cheats needed to make most optional objectives easy.
I'd like to mention that you have to load up a memory & then activate the cheats required, you have to do this every time you load a new memory.
The cheats needed are:
- Immortal (god mode)
- Cornucopia (unlimited ammo for Shay & the Morrigan)
- Fairy Gunpowder (weapons don't have to be loaded)
- Gray Men (Shay never gets detected*)
- Master of the Wind (the wind is always at the Morrigan's back)
- Burn Them! (Burning oil burns hotter, and causes more damage)
You need to finish a total of 30 Abstergo challenges to unlock each cheat periodically.
(Keep in mind that the game doesn't save progress once you activate cheats, so you'll have to do this in one run.)
TIPS & INFORMATION
If you mess up (e.g. get caught, or take damage) you can always load from last checkpoint in the menu.
I was initially having trouble with assassinating Le Chasseur without taking damage, but soon realised that you can simply shoot him as soon as you load in.
To kill Kesegowaase with the puckle gun, I ran straight ahead to the nearest puckle gun and he ran straight towards me for an easy kill. I was able to get the objective done in 1 attempt without using cheats, but I've heard some say it takes several.
*It seems that the Grey Man cheat doesn't work the way it's described in-game, you will eventually get detected if you stand out in the open for too long / kill an enemy in the open / get attacked by a stalker. I found it best to silently take out enemies from rooftops with the sleep darts & kill nearby stalkers, this is made easier when you're using the Cornucopia cheat.
Hopefully this is enough information to make getting this achievement easier to grab. If you have any problems be sure to let me know before casting a negative vote. Thanks!