Not to disparage anyone, but I originally got into 1/6 with military figures from Soldier Story, so the Sandtrooper was actually a refreshing, fun, easy assemble. Be careful with his backpack - two things popped off mine that I had to glue. Also, be careful with the shoulder holster - I would take off as much arm armor as you could before trying to slip it through. I have had elastic straps like that rip on similar 1/6 figures. The whole figure shouldn't take you more than 20 minutes to put together as long as you take a little care. .
Also, as long as you tighten the straps down well enough, you shouldn't have too much issue with the backpack moving around. Gluing shouldn't be necessary.