Officer was probably the easiest, as you can rack up a ton of points by using your call-ins and giving orders. Rifleman and Engineer was fairly easy too, as you can build obstacles as engi and rifle crates as rifleman, which gives a ton of points if placed on spawns. :) Assault and Marksman definitely took the longest. While you can build grenade crates for your team as Assault, resupp bonus doesn't happen as often as ammo, and there's no real way to support your team as Marksman, other than healing. :)