Making armor invulnerable
It is possible to create nearly invulnerable mechs through an expensive and deadly process.
First, put a computer controlled ally in the desired mech, go on a mission (preferably a capture or destroy mission so you can take your time doing this; multiple-round missions negate the entire process as field repairs take place) and destroy the left, center, and right torso sections of the mech. When the mission is over only repair the arm, head, and leg sections of armor, and the internal structures of everything. Repeat the process until the armor in these damaged sections is completely destroyed. Eventually, while the internal structures will be fully functional the armor in these sections will appear to be destroyed, making them impervious to damage.
This process is highly expensive in both money and lives. Several pilots may die in the process of making a mech nearly invulnerable. In addition you cannot make the legs invulnerable, as no matter how many times they are destroyed (and the internal structures are repaired), the game always reports the armor as being severely damaged to within paper thin levels. Arms cannot be made invulnerable as they are the first items on a mech that are repaired and legs are second. In order to repair any armor damage to the legs the arm damage must be repaired first.
It is possible to render the cockpit invulnerable, however due to the immediate death of the pilot in control of the mech, this modification has the highest cost in lives. In addition, computer-controlled allies piloting a mech with an invulnerable cockpit behave in an erratic fashion and flee for their lives whenever they are assigned to the mech with the special cockpit. If you want to create a mech with an invulnerable torso and cockpit it is advised to create only one (the Battlemaster is a good choice).