Darkest Of Days

After missions, you will receive "Upgrade Points" that you can use to upgrade your Pistol and Rifle while at the KronoTek Briefing Room. Each full upgrade costs 10 points, so each weapon costs 40 points to fully upgrade.

I've read that it's impossible to get 80 upgrade points in a single playthrough, so my advice would be to save all points until you have 40, then upgrade the pistol, get the achievement, reload the prior save, then upgrade the rifle (as it will help you more later on than the pistol will.)

The maximum number of points available in one playthrough is 48 (3 points per level), so you will not be able to get both this and Rifle Upgrade in one go, unless you do as fisher81 suggested and save up points, fully upgrade, and reload.

Additionally, remember that killing Blue Aura Soldiers decreases the number of points you receive. It seems that killing even one will decrease your reward to 2 points in a level. If you accidentally shoot a Blue Aura Soldier, die and reload. If the Blue Aura Soldier is killed by anyone other than you, it does not count against you.

If you end a level and receive less than 3 points, reload a save and replay the level. Otherwise it is entirely possible to miss these achievements and have to play an entirely new game; there is no way to replay levels or gain addition points if you fall short by the end of the game.