Hello and welcome to PBreezey's guide for The Division achievement "On the Level"
To unlock this achievement you will need to level up an Agent to level 30 (which as of the time of writing this is also the level cap). To level up you need to gain experience (xp), and there are a number of ways to gain xp:
For each of the following methods, ensure you are the appropriate level for the area of the map you are in. If you are not, you will likely just die a lot and not actually earn much (if any) xp.Killing Enemies: Each enemy you kill rewards you xp. You don't get a lot of xp for each kill, but xp is xp.
Accolades:You can also get additional experience for kills by earning accolades. Accolades are little xp bonuses for killing enemies in specific ways: headshots, multikills, etc. There are also accolade bonuses for surviving fights, reviving teammates, and I am sure there are some I have yet to discover. Again, the accolades aren't big xp boosts, but they are xp.
Collectibles: The Division has a decent amount of collectibles scattered throughout the world. For each collectible you pick up, you get a pretty sizable chunk of xp. Since there are
so many collectibles, were you to collect all of them before hitting level 30, they would account for a decent amount of your total xp. Additionally, picking up collectibles will also help you work towards:
Encounters: Encounters are located in each area of the map and are associated with a specific wing of your Base of Operations. You can find encounters by interacting with the Notice Board in each Safe House. In addition to awarding you xp Encounters also give you supplies, which assist you in upgrading each wing of your Base. Though Encounters don't give as much xp as Side Missions or Story Missions, the bonus of supplies makes completing them worthwhile so that you can also work on:
Side Missions: Side Missions are located in each area of the map. You can find Side Missions by interacting with the JTF Officer in each Safe House. Side Missions award you more xp than Encounters, but still less than Story Missions.
Story Missions: The biggest chunks of xp you can earn in the game come from Story Missions. Story Missions are automatically on your map (except for one) and are marked by large emblems of the wings of your Base of Operations. Though they take longer, each mission will result in earning large amounts of xp, loot, and (a lot of the time) other story related achievements.
Boosting: I wanted to separate this one out from everything else as it takes a little more explaining. You can boost xp by if you are a lower level by playing with friends who are a higher level. I did this with some friends and was able to help them level up very quickly. When I did this, I was level 23 and my friends were level 9. Even though they invited me to their game, the enemies in the game all set to level 20 because of my level. Because of the high level, my friends were earning 1600 xp for every kill I got. Now, my friends weren't able to do much other than hide; but like I said, they were getting large amounts of xp for kills in addition to the normal large amounts of xp for Side Missions and Story Missions.
My experience: When I got level 30 I had completed all Story Missions and had completed all Side Missions and Encounters in all but 3 areas of the map. As far as the collectibles, I wasn't going out of my way to pick them up but if I was close to them then I would grab them.
If you have any questions or comments please let me know. If you're going to downvote, please comment first and give me a chance to improve whatever issue you may have.
Good luck and happy hunting!