The achievement description is a bit misleading as you don’t have to acquire 75% of the points in the game (a silly notion when you consider that you can technically acquire an infinite amount of points), but rather you’ll earn this achievement when you fill either your Paragon (blue) or Renegade (red) bars up to 75%. There’s plenty of points to go around, but if you want to ensure you get this, you’ll want to complete as many assignments as possible (making this another achievement you’ll likely obtain during your hunt for Completionist) and whenever you get the chance stick to either the top right (Paragon) dialog options or bottom right (Renegade) dialog options and pick Charm (blue) or Intimidate (red) responses whenever they pop up. To ensure you have access to Charm/Intimidate choices (and hence, can gain the Paragon/Renegade points from selecting them) you’ll need to invest some talent points into Charm/Intimidate.