This is not difficult as long as you take your time. I completed it on General (hard) difficulty. But go ahead and put it on easy if you want.
*Warning*
There is a rumor going around that this is possible to do with your other troops as long as you don't capture the points with them (use only snipers to capture the points, keep your units outside the area), I tried this, and it didn't work. I retried the mission using ONLY snipers (except for the first intro sequence) and got the achievement when I captured the last point.
- Try to always keep your snipers in green (full) cover when fighting, they will rarely get hit.
- Use the suppression ability when fighting two or more enemy squads.
- Save after every encounter, things can go wrong very easily.
- Use flares if you suspect enemies up ahead, your rifle range is longer than you LOS.
- After clearing the first outpost, you CAN capture the half-track with your soldiers. Use it to replenish your snipers if any die (not the squad, just the sniper. If you lose a squad load previous save).
- On the last outpost, I successfully attacked the top side. Use hit and run because of mortars. Check closest building for garrison, take 2/3 shots and run back. repeat until clear. When your ready, bait the troops guarding the mortars, back off, wait outside of mortar range, and kill. When it's just the mortars, get as close as possible and kill them.