In the Secret Identity menu there is an option called "Accept promotion".
This will be locked when u start the game for the first time and will remain locked untill you reach lvl 100.
At level 100 you can accept your promotion which will net you the achievement
Once you've accepted you'll automatically become lvl 101 and the menu will be locked again.
You can accept a promotion every 100 lvls up untill lvl 1001.
For every promotion you'll get a different grade, and calling card background.
Lvl 501 and lvl 1001 will also unlock a mascott.
The amount of experience needed will also reset when accepting a promotion; for example getting from lvl 101 to 102 takes the same amount of XP as getting from lvl 1 to lvl 2 or as getting from lvl 701 tot lvl 702.
After accepting your first promotion you will also be able to do the Legendary Feats of Prowess
When you accept a promotion you will lose all your weapons, mods, funfacts, psych profiles, support items, and gadgets.
You will only keep weapon skins, mascotts, calling cards and costumes.
Therefore it's better to get the other achievements like "Gearhead" and "Weapons Specialist" before you get this one.
Feats of Prowess (challenges) will also be reset when accepting a promotion, with the exception of the "Lifetime Feats".
This means you don't have to get the Nemasaurus achievement before acepting a promotion as the counter will not be reset.