For this to unlock you must not be spotted by anyone in the level, which includes Pioneers, Pirates, and Bandits. You are ALSO not allowed to engage them in any way – you can’t knock anyone out either. Killing animals and monsters is fair game.
Every location that you need to get through has multiple pathways that let you sneak past unnoticed so there is no definitive guide as such. You want to make generous use of quicksaves and lures (cans) to draw enemies away from you. It goes without saying that you must always always always wait for nightfall before trying to sneak past an area with human enemies in it. You will find a bunch of campsites between them which let you wait, so you use them whenever you get the chance.
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You CAN be spotted by certain “friendly” enemies – for example at the first pioneer camp to the very left there is a lone sentry chopping wood. As you get closer to him he will hear your footsteps and turns around – this is a scripted thing – and try to surrender, but if you don’t attack him (not even knock him out) and lower your weapon he will tell you that you can bypass the sentries by taking the caves right ahead of that area. This, as well stumbling on a sentry that was told to stay in the corner in one of the huts on top of the larger pioneer camp (the one where the meeting between the pirates and pioneers takes place), is fine and won’t void the achievement. What matters is that you 1) DO NOT ATTACK ANY HUMAN ENEMIES and 2) DO NOT GET SPOTTED BY ONE SO THAT THEY ENGAGE YOU IN COMBAT.
Don’t risk it by trying to save any of the captives in the bandit camp – just by clearing areas without attacking anyone you will have enough positive karma to get the Alyosha achievement.
The most challenging part is to sneak past the last pirate camp before the sawmill because you need to use a boat to do it. You need to sneak into the camp and raise the gates for you to swim through, the lever controlling the gates is on the second floor of the shack immediately to the left of the gates. Make sure you save before you try to swim out of the camp. There is a boat in the camp itself, but make sure you extinguish the lantern that’s on it before you get in if you elect to use that one. Also make sure you don’t go at full speed when you are in the boat – hold RT about halfway because you will be swimming through the middle of the camp for about 10 seconds and you don’t want to make more noise. Once you clear that camp and get to the sawmill (it’s the location with all the glowing mushrooms, toxic gas, and shrimp monsters) you will fulfill all the requirements for this achievement as there are no more human enemies past that point, but the achievement won’t pop until you finish the level.
See more: Forest of Illusion 4