To get the Veterans cheat, you need to have around 65 Abstergo challenges completed. The Veterans cheat makes it so guards do a lot more damage, so stealth is the better approach for this. Supply camps are the wagon-like icons on your map that are located in the River Valley.
I recommend you do this after you have already captured 10 or more supply camps to also get
To check it, go to Database -> Statistics -> Misc. Statistics -> Total Supply Camps Captured.
1. Turn on the Veterans cheat.
2. Go to the River Valley and start raiding supply camps.
3. Once you've raided 10, the achievement will pop.
I don't know why it says "large supply camps," but I managed to get the achievement when I raided 10. Also, I did this without dying. I don't think that's a requirement for this achievement, but considering the other cheat achievements have that requirement, I didn't wanna take the chance.