From what I know of the Lambda drive is that it's basically Tech Telekenesis, Aka, a device that allows the wielders willpower to become a physical construct
This is pretty close.
My own understanding of the Lambda Driver is, it's a semi-organic device that takes the mental imagery of the pilot, and transforms it into an energy form with physical dimensions.
Example one: If the pilot were to see two projectiles coming at him at high speed, and without the possibility of dodging them, the Lambda Driver could take the image of a shield or armor, and create a barrier to those incoming projectiles. (See 'Full Metal Panic: The Second Raid' episode one)
Example two: In the hands of a particularly skilled pilot, the Lambda Driver could, theoretically, be used to extend kinetic-energy pulses inside the cockpit of another mecha, with devastating results (Seen several times, at the hands of Gawron, in the first season of MFP).
While I'm not a god-modded by any stretch, I'd like my use of the AXR-7 Arbalest to be without any major restrictions on the Lambda Driver (I.E, my pilot knows how to use it at will.). As I said, I'm not a god-modder, so it wouldn't be abused.
Insofar as the 'Achilles Heel' aspect, I can see a much more realistic set of issues facing the Arbalest. First off, it's got a limited endurance during any one deployment (Stated as 100 hours). Second, it's primary weapon, the Boxer Shotcannon (Think of a mecha-sized shotgun filled with slug rounds) has a VERY limited amount of ammo in it (Due to the size and construction of the weapon, I'd call it a six-round magazine, at best). Finally, it's head-mounted vulcans eat up ammo at a very quick rate, leaving the last-resort weapons as an armor-piercing dagger, and another knife called a 'micro-molecular cutter' (Basically a large knife with a chainsaw-like blade on it's edge).
Does this help at all?