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Sequence 6, Memory 1, Gatekeeper.
*** Spoiler - click to reveal ***In this mission, you're supposed to hold off the French soldiers by lowering the barricades at three gates. Ignore the objective. Instead, run around the gates to lure the soldiers so that they chase after you. Once you gather a large amount of them (you'll see a bunch of red arrow indications on the screen), stand still, and let them crowd you, Hold to use Arrow Storm, and you should kill at least 10 of them to unlock the achievement.
To have a higher chance to hit 10 with one assault, it'd be advisable to let a few soldiers attack you, and you to take some damage. This means more soldiers are surrounding you, giving you a higher chance of hitting 10 with one Arrow Storm.
Do note that you'd need to have 3 red bars fully filled up to be able to activate Arrow Storm.
If somehow you didn't hit 10 with the Arrow Storm, you can restart the memory to try again. Note that you'd then need to wait for the red bars to refill before you can use Arrow Storm again. You can do so by simply running around without engaging in fights. It'd take some time though.
You can replay this any time by going to the pause menu, selecting DNA, and Sequence 6 -> Memory 1.