Play Freeplay Mode on the Security Tower map with 1-on-1 settings (Team Size should be set to 1, as should Max Team Size). Start as a Resistance member of the Soldier class and blow the door to access the next mission objective, then change to the Operative class. Go toward the safe you are supposed to hack, making sure not to be killed by the security agent. When that security agent/engineer tries to remove your hack box, position yourself on the opposite side of the glass. If positioned properly, your adversary will spray ineffectively and you will gain experience due to the "hacking" and "guarding" functions you are performing. As long as you aren't positioned too close to the safe, you'll be stuck in a stalemate with your opponent and will earn experience the entire time. Then, when you're ready, move closer to the safe to complete the mission and receive your points.
To use a heavy gun with a small body type, change to a heavy body type and select a gun (make sure it's exactly how you want it as you cannot edit it afterward). Choose any gun of any weight. Now, do the same for your secondary and make your default weapon. Go change back into a medium/light body type. Your default guns will stay the same, but your body will not, thus, when you play matches, you can have a light body type with a gatling gun.