In the multiplayer section you can upgrade your solider's equipment by purchasing Tier 1-5 items to enhance their abilities.
Tier 5 is unlocked when you reach level 90. Level 90 will definitely take some time to get to, however if you refer to my solution on the "Simply the Best" achievement on hitting level 100 in multiplayer it lays out a method on how to power level up to 100.
The achievement will unlock after you purchase the tier 5 item and enter the next match of multiplayer. Bronze, silver, or gold does not matter as long as it's tier 5.
With the recent patch in place it is now level 90 to accomplish this achievement.