By the end of your second playthrough you should have enlisted quite a few pawns, but most likely not enough for this achievement. You should work on this over the course of your playthroughs so it isn't so much of a grind in the end. You will want to constantly do this to have the highest level pawns you possibly can.
To enlist a pawn either find one walking around, speak to it, and enlist it, or enter The Rift by accessing a Rift Stone. These are located all over Gransys. There is one in the Pawn Guild in Gran Soren. Clear your party by pressing to speak to a pawn who is with you. Choose 'Release' for them to leave. To avoid going through the same tedious process of selecting a gift, rating, and choosing a message to send with the pawn, go to the options menu and select 'Offline Mode'. Now when you hire 'offline pawns' you won't need to send gifts and messages with them. This will likely cut the time it takes for this achievement in half.
You can enlist the same pawn and it will count towards the achievement. Just enlist 70 times and the achievement will unlock. Your progress can be viewed in the Adventure Log.