This page will cover the achievements that don't really fit anywhere else. Some you'll get just by playing the game, while others require a bit more work. None are overly difficult, however.
From time to time you'll see English prisoner-of-war ships sailing around, which hold 100 POWs each. If you defeat the two ships escorting the POW ship, then you'll be able to board it for a ton of resources and you'll save the prisoners inside. The amount of prisoners remaining depends on how much you damage the ship; try to avoid firing at the POW ship so your cash reward is greater.
One achievement asks you to save at least 300 of these prisoners. The earliest you can do this is after Sequence 1-3, but it'll be easier to do the stronger the Morrigan is. The minimum amount of POW ships you need to board is three, with 100 each of course, but the odds of you not damaging any of your first three ships is going to be all up to how precise you want to be. It usually takes people four POW ships to earn the achievement.
During Sequence 3, supply camps will show up on the map. These contain supplies which can be viewed by hovering over them, and are pretty convenient given the small amount of effort you need to put into getting those resources. Looting 20 of these gives you another achievement, and you can find more supply camp locations by paying for tips at a bar in River Valley. This might not be necessary, however, because if you just keep hitting up supply camps then the ones you've previously been to should respawn as a result of a lack of active ones on the map.
Like supply camps, ship convoys also hold lots of materials, but are found in the North Atlantic. These are a bit harder to spawn, but there's a trick to it that makes it easier. Fast travel to Anticosti or Fort Louis in the North Atlantic map and look around for a set of three ships on the map itself. Then just take it down by either boarding it or sinking and looting it for it to count towards your total. You can quit out of the Animus after you board/sink the convoy to avoid having to escape battle and build up wanted meter, which will make this process that much faster.
When enemies use smoke bombs, put on your gas mask when prompted to prevent being affected by it. You just have to do this 20 times for an achievement, which is very simple and won't be an issue since gas bombs are frequent.
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Counter 20 smoke bombs successfully using a gas mask
During Sequence 3, you'll be introduced to stalkers, who wait for you to get close in order to land a strong blow. There are two general types of stalkers: those that attack from your level, and those that jump down from above. Countering and killing stalkers of both types grants two achievements, but you need to be using the hidden blade when you're attacked to do a proper counter-kill. If you're using your sword and dagger, then it won't count for whatever reason.
Counter 15 air surprise attacks
When you finish Sequence 3 and get the grenade launcher, you'll be able to tackle these next two achievements, which require you to affect five enemies at once with both the sleep grenade and the berserk grenade. Simply fast travel to the viewpoint in East Village, New York, and fire a firecracker dart into the crowd of British soldiers to the southeast. When they're all clumped together, lob a sleep grenade at them first; that way you can do the same with a berserk grenade without having to worry about them moving around.
Put 5 enemies to sleep with the sleep grenade (at the same time)
10 guides Instant Vikings
Hit 5 enemies with the berserk grenade (at the same time)
Two easier achievements come from sailing, and only require you to break 100 icebergs (the large chunks of ice that cause ripple waves) and to carve through 500 meters of ice sheets.
Finally, by fixing all the computers at Abstergo, you'll get a bunch of lore and an achievement for your efforts. The tablets scattered around the place don't matter for this achievement and you'll only be able to get to all of them after you beat the game, so don't worry about this until you're nearly done with the game. The puzzles aren't too challenging and will only require a few minutes at most when they get more difficult.
Repair all computers in Abstergo Entertainment
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