The achievement requires you to purchase each of the items in the armory atleast once. The armory is marked by a Grenade symbol on the map and on your screen. You do not have to buy everything in one match this my be completed in mutiple matches.
For this achievement you are required to be the rank of 37 in spec ops this takes 939,600 xp. This is due to the fact the riot shield doesnt unlock till 37. Spec ops ranking system is seperate from Multiplayer so any rank in mutiplayer wont be required.
What you have to buy-
Body armor 2000
Self Revive 4000
Sentry Gun 3000
Sentry Grenade launcher 4000
Riot shield 3000
Total- 22,250 points.
Things you should know you can not have two sentry guns or one sentry gun and one sentry lancher at the same time you must place one on the ground before being able to buy another. The self revive may also require you to buy another perk or get downed atleast once before buying it this is due to starting the game with it when you unlock it. If it does this can be done easyly with one enemy left in the wave or buy another perk from the air support armory marked by a airplane on your screen.
Easy ways to rank up -
Missions - You can play each mission in spec ops on veteran by yourself and gain upwards of 33,000 points a mission completed for the first time.
Survival - Doing the first survival map by yourself can get you quite a bit of xp from kills but may take a while.
Grinding - Have found a method that can get you anywhere from 4000 to 8000 points in just one minute.Set difficulty to veteran(or can put it on any difficulty you prefer), on the first part of server crash you start in a car. You can run through this sequence and destroy each car netting you 700 or more points per a car destroyed. Will try to make a video if call of duty elite allows it , of a speed run through this method and where to shoot to maximize your point gain. Here's a video showing the mission in the mean time.
Easy ways to gain cash in survival -
Please look at my solo method for the Get rich or die achievement (get 50,000 points in one match) to help you get money faster to buy each item.
Please thumbs up if this helps and if theres anything wrong comment and ill try to fix it Thank you!