The maximum agent level is 20, which can be a daunting task. Here is a breakdown of the experience required at each corresponding level:
Level 2 – 100
Level 3 – 200
Level 4 – 400
Level 5 – 800
Level 6 – 1600
Level 7 – 3200
Level 8 – 6400
Level 9 – 12800
Level 10 – 15400
Level 11 – 25k
Level 12 – 40k
Level 13 – 53k
Level 14 – 64k
Level 15 – 78k
Level 16 – 90k
Level 17 – 99k
Level 18 – 100k
Level 19 – 100k
To help with this process, the 'Gifted' perk can be quite helpful (+5% experience for all tactical missions) but only really at the higher levels as the extra 5% in Chapter 3 is garbage. In addition, the 'Loyal' perk is very handy too since it is possible to lose an agent that is not loyal to random events in the mid to late game. This is particularly true of the extended campaign where I lost a level 16 agent with max stats who actually was loyal from an event related to the extended campaign. Chances are you will be able to finish the game before any of your agents reach 20, in which case just reload before the final mission and grind tactical missions.
Another option is to buy the highest level agent possible (Level 14) during the final chapter and boost them with tactical missions. I used this method since none of my agents were above 13 when I reached the end of the game.
Since this process can be time consuming, I recommend going for:
and
on the same play-through.