From Space

Max player rank is rank 26: Zapper

You rank up by going to the character selection menu before a mission, when any of your characters are level 5 or higher. Press to rank up.
It will reset all stat upgrades gained when leveling up that character, so it is advised to rank up as soon as you get to level 5.

I had issues ranking up in lobbies with another host, lost 2-3 rank ups somehow, so to be safe leave the game to rank up in your own lobby. If you are in a good lobby where each veteran/specialist mission is a breeze, you can also switch character and save up several level 5 characters for rank up later if you don't want to leave the lobby just yet.

A faster method to rank up is to join games in progress, preferably at the final 1-2 rounds, and help other players complete their missions :)

2-3 veteran missions should be enough XP to reach level 5, depending on the amount of players successfully extracting and number of objectives completed.
It is also possible to go straight to lvl 5 in 1 specialist mission, if enough bonus XP is gained.
The XP is kinda low on the lower difficulties so more missions are needed there.

This particular achievement can be an absolute slog & tedious task to level up in total of 130 levels to go through all 26 promotions to "Zapper." I'll share with you some information that I learnt & gathered during my experience of it.

There is ways to complete the first map "Octogon" within the 12 minute range on "Specialist" difficulty. All it takes is map knowledge of knowing which method to use & how to execute it. Discovered that you can manipulate the spawns for maximum efficiency.

Once you find the sweet spot(s), recommend things you'll need for this:
● Barb Wire for slowing them down.
● Turrets for helping killing them.
● Adrenaline Pack for fast mobility for objectives & a synergy with 2 perks.
● Dispenser for limited travelling & keep them spawns active at max capacity.
● Dome Shield for the defend objectives & blocking off pathway enemies or spawns.
● The perks, "Adrenaline Enhancer" & "Hack Off" for stacking the damage multiplier to 4.5 + 2 for the ballistic weakness.
● Weapons with large capacities like the LMG or Flamethrower.

With 2 - 4 people (depending on XP gains & objectives completed) you can get 5 levels needed within around 25 minutes. Give or take a minute or 2. So, sum it all up. It'll possibly take you around 10ish hours to get from "Adventurer" to "Zapper."

If you guys have any questions or want an helping hand when it comes to this. Please let me know! 👍🏻 My GT's: Whizkyman