Sniper Elite 5

To reach Rank 40, you need 545,000 XP. Ranks 1 through 35 require 13,000 XP each and ranks 36 through 40 require 20,000 XP each. You earn it by completing objectives, both main and optional, completing kill challenges and killing enemies in a variety of ways (organ shots, high ground, empty lung, sound masked, ghost, etc.). The campaign and Survival/multiplayer have separate XP progression, but reaching Rank 40 in either will unlock this achievement.
 
Ranking up will come with natural campaign progression, just as it does for all the cumulative achievements. By the time you’ve completed the campaign the first time, you will probably have reached Rank 25 or so (depending on if you played solo or in co-op). Replaying missions to farm the cumulative achievements and playing the campaign again on ‘Authentic’ difficulty should raise you to the required rank.
 
As mentioned above, playing Survival and multiplayer progresses separately from campaign, so unless you’re playing a lot in those modes, ranking up in campaign is the way to go.
 
A good way to grind out remaining XP is to use the “Save Game” method. Completing all optional objectives in a mission gives you 500-1000XP and you earn 1000-1500XP or more for completing all primary objectives. The best way to rank up is to save your game just before you complete the final optional or primary objective. Save your game, complete the objective, then wait for the XP to register on screen before reloading your save. The XP earned is instantly saved so you don’t lose it with each reload. Keep repeating this as long as needed to get to Rank 40.
 
To check your progress, press then navigate to Service Record > Medals > General and look in the first section to see your medal progress.