The required amount of XP in total is 100,006.
Each level is only 1000 XP, so 1000 x 100 = 100,000. the 6 points are allocated to level 100 itself. I don't know why, but you need that little amount of XP in order to Re-up.
Once you have reached the required amount of XP, simply head to the main menu and hold
. Your level in the top right-hand corner should also have a prompt. The achievement should pop as soon as you've Re-up'd and you're back to level 1. It's a shame you don't get anything cool, or there's anything to display your prestige once you Re-up--but at least you're 5% closer to Re-up 20, I guess!
In terms of levelling up the quickest; Escape and Horde are your best bets. I managed to go from 1-100 in about, less than 10 hours of game time, so this achievement isn't very hard in the grand scheme of things.
I should also note that XP boosts can be purchased from the store to help you level up even quicker, but they require Iron (In-game purchasable currency).
I chose the Escape route of levelling up, which is really quick! You should level up 5-9 ranks per game. Each game will take between 10-30 minutes to complete. I'll try to outline the quickest way of doing this, but I'm going to assume you've played the maps and modes and know the basics.
What you wanna do is play on Elite difficulty or above (do not play on Master). Character doesn't matter, but I chose one of the default 3 characters (Lahni, Keegan and Mack). The lower the difficulty, the quicker you'll finish the level, but the higher the difficulty, the more XP you'll receive.In terms of character cards:
make sure you choose ammo and damage related cards, as ammo is sometimes scarce, and enemies can be hard to kill. On higher difficulties, try and focus on one enemy at a time, instead of multiple, as they can recharge health which is very annoying.In terms of bosses:
always trap them in supply rooms (the rooms with ammo and weapons), however, if you have Lahni in your group, get her to use her special ability on the boss (only works on Warden) and she should kill him in no time as it's a very powerful ultimate.In terms of other players:
I would always recommend finding a group that communicates, but your milage may very when it comes to randoms. Using the LFG section on Xbox is great, but TA sessions also work like a charm.In terms of maps:
I liked the maps with a checkpoint in the middle, as they give you more opportunities to kill enemies. Also, if you're looking to finish the map as quickly as possible, many areas can be skipped by simply running past everyone until the end.
All-in-all this achievement is quite easy and you'll more than likely earn it way before earning many of the other online achievements that are scattered throughout the game. If you have any questions or tips that I've missed out, please let me know!
Anyway's, thanks for reading my guide and happy hunting everyone!