Edit: 3/19/21 - Possibly buggy achievement as reported to not unlock at level 25.
Relatively easy achievement, if not a little repetitive/ time-consuming.
Each day that you play this game, make sure you start off by accepting/completing the daily faction missions from both SHIELD and the Resistance. Also completing the mission chains that deal with each faction will give you bonus XP as mission rewards.
Again, not a hard achievement, just takes time to accept & complete the various daily missions.