Supposing two 'mechs stand on level-0, if one punches the other, this one hits on the punch location table, which contains no leg locations. So, this indicates that a 'mech cannot punch to level-1 (where the legs are) and must punch to level-2 (where the torso is).
Now, if a mech were to try to punch a tank and if both the mech and tank resided on level-0, the tank would be at level-1 (the same level as a mech's legs). So, this appears to indicate that a 'mech cannot punch at tank. I, however, do not see any textual evidence to justify this conclusion. Do we have a RAW answer for this?