Spend 25 Attachment Credits with each role
You get credits for both sides, Ghost and Bodark. Just stick to one class at a time and just unlock attachments for the weapons you unlock and have. You will need to get the Scout, Assault & Engineer to at least level 13 each, to have enough credits. The Mod Pro achievement will most likely be unlock for some at this point.
Note: Pistols also count towards this achievement. I'm not sure if there is a way to track how many attachments you have bought for one class.