There's a total of 32 weapons and equipment you'll need to unlock to obtain this achievement. These are all unlocked as you rank up through the multiplayer.
You'll unlock the last lot of stuff at level 40 and that's when the achievement will unlock. You don't have to purchase them.
The weapons unlock at these different ranks:
Rank 1: PT92, M500, .38 Revolver, M10 & MPK.
Rank 2: Flashbang.Rank 4 - 1911, SAF .40, M972, FAL, AK-47, Mini-30, M4 Super 90, RPD, Tin Can & Grenade.
Rank 20: 608 Bull, SPAS-15, FMP G35, MD-97L, AUTO 9MM, RPG, Smoke Grenade & Tear Gas.
Rank 40: Super Sport, MICRO 9MM, DE .50, G6 Commando, Grenade Launcher, LMG, M82A1 & Sawn Off Shotgun.