I see that is has a BAB of +11 under statistics, but under offense it doesn't say anything about any kind of melee attack. Are we supposed to assume that it has +11/+6/+1 BAB and then subtract (because it only has 6 str)? or given its low Str are we to assume that it doesn't melee at all and therefore doesn't make AoOs?
Something to note is that it has a reach o 0 feet so something would have to be its square to provoke. That said, if it were to attack in melee I would assign it an unarmed strike equal to its size.
It has a BaB because all creatures have a BaB. What it doesn't have is a weapon. Technically it can make an Unarmed Strike (Headbutt), but since Demiliches don't (by default) have Improved Unarmed Strike, they do not threaten with Unarmed Strikes, and thus cannot AoO with them.
Its theoretical attack bonus with this kind of attack would be as usual. BaB +/- Str or Dex modified by any random modifiers it may have.
I forgot that it was a tiny creature, so it doesn't threaten any squares other than its own anyways. thanks for the reminder guys!