To try and make the Wasteland robots "weaker" there was this added...(I actually cant remember if it is from one of my upteen billion attempts at adding new bots...or Deon putting in something to kill my "invincible army" because as you've found out, even made out of copper they're nigh-indestructible)
[BODY:CPU]
[BP:CUC:CPU casing:STP]
[CON:UB]
[DEFAULT_RELSIZE:300]
[BP:CU:central processing unit:STP]
[CON:CUC]
[THOUGHT]
[INTERNAL]
[SMALL]
[CATEGORY:BRAIN]
[DEFAULT_RELSIZE:100]
note that the "casing" isn't internal, so it's basically an access panel somewhere on the upper body with the CPU right underneath it. The tissue for the case is usually either thinner than the other tissues, or is made of a easily fracture-able material like glass. A lucky shot with a rifle, or a targeted melee attack can kill the biggest, baddest robot in one hit.
There is also the option of adding hydraulics & pumps (...or maybe lines and E-tanks?) as a circulatory-system analogue. Allowing them to "bleed out" (give the lines and tanks a high healing rate so only repeated cuts could kill the bot)
You mind If I try and cobble together a mechanic caste based off of Auto?
http://megaman.wikia.com/wiki/Autofigure I can give them interactions to repaint and replace limbs...
...unless you want to do it yourself as part of learning process?