The goal is to tighten the nut without rotating the shock internals, so you can't just put a 21mm deep socket on a torque wrench and go to town. You need an allen wrench on the top of the strut to keep it from rotating.
With two people, it's not hard. You can put a 21mm combo wrench on the nut and put an Allen socket on a torque wrench. Have someone hold the shock, and hold the torque wrench steady with one hand. Pull the combo wrench to tighten the nut till the torque wrench clicks.
Another method I've done is to put a deep 21mm socket on the nut, an Allen through the socket, and a pipe wrench on the socket. It can tear up the socket, so buy Sears and use the lifetime warranty.
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