This is more a tip than a guide: since you have to play at least 12 weeks to get this achievement:
The A Reservist achievement in For Honor worth 25 pointsManually deploy Troops in 5 different Campaigns.
I would suggest to play every day 1-2 hours and complete the orders and contracts
(play X matches, perform Y takedowns, capture Z zones while playing Dominion matches and so on...), in order to maximize the XP and don't getting too much bored.
If you saved some steel, you can buy the "champion status
" (main menu -> store -> champion status), that gives you 40% more XP at the end of each match. A good offer is 15k steel for 1 month of "champion status", especially if you find the some special offer (e.g. 30+30 days).
Moreover, if you have just started the game, while playing with a Wu Lin Heroe you can also unlock this achievement