'Not the glass, and even if the metal doesn't dent the shock can transfer to the lens.' No, that's not how armour works, if impacts were carried into the cockpit, Higan would die from anti-tank rounds, the force is deflected away from vitals, that's literally the entire point of armour of all kinds.

'The rock does not need to hit the lens directly to fuck it up' Yes it really does, as I keep referring to the reference picture the armour plating on top of the lens is literally designed for this very purpose, if it could be completely destroyed by a monster with a rock bashing on top of the armour it would be the least combat applicable lens in the universe. It's also nice that you can choose to ignore attacks until convenient, such as when Slimy carried out his rock attack in the face of a grapple while being thrown about by the Mech, dodged it, and then tried to dive through the arm. 

It's not really my fault you chose an ineffective attack. Slimy presumably has long sharp claws or could have picked up a bone from the ground, something sharp and thin with which to by-pass the armour and ram directly into the lens, you essentially brought a hammer to surgery. 

Actually, I decided that the right of the lens would be damaged based off the reference picture, you should check it out. You obviously haven't yet. Though the damage is really no more convenient than

1. Slimy predicting the Mech's wall/scrape defence and immediately moving to the outside of the left leg, counter to its previous movements of grappling the right leg and diving underneath the Mech.
2. Slimy having a half eaten corpse ready to heal all damage caused by the expenditure of over half the Mech's ammo.
3. Slimy having an intimate knowledge of battle mechs and their weak-spots, despite its echo-location making it highly doubtful it could discern the lens at all. 
4. And finally, Slimy's infinitely changing body size, that makes it capable of waltzing around the Mech without anything pesky like logic getting in the way.