Battlesuit could handle it.
Exactly. That's why Chainsuits are so efficient/ridiculous. Whether it's a Gatling (manually-operated multi-barrel) or Mini (battery-operated multi-barrel, making it the more likely candidate), they're able to use a weapon that would otherwise need to be mounted to a vehicle to fire effectively. But, of course, that's why they're called Battlesuits.
And Mass Driver being some form of futuristic Rail Gun also makes a lot of sense. I always imagined that, in the world of Trem, the Mass Driver was invented as a more efficient answer to the sniper rifle. If you've ever fired a real gun (or at least played a lot of Counter Strike), then you know that bullets are only accurate to a certain range. Even rounds designed to travel great distances, like those from a high-powered rifle, still require extensive training to be used effectively. Basically, a bullet's accuracy has a lot to do with its
mass (Ah, there we go...)
Now a Rail-Gun on the other hand uses electromagnetism to basically negate the mass of its projectile, allowing for a MUCH GREATER range of accuracy.
For a frame of reference, here's a quote from the Rail-Gun Wiki:
"The U.S. Navy has tested a railgun that accelerates a 3.2 kg (7.0547 pound) projectile to 2.4 kilometers per second (seven times the speed of sound)."
If the Mass Driver is, in fact, a rail-Gun then its rounds aren't actually mass-less. Their mass has simply been negated by the gun.