Updated.
Should we give them Improved Sunder as a bonus feat? Or some form or armor-destroying ability like a bebelith (usable only against those trapped in a pitfall)?
I think a special attack's a better fit than Sunder.
Hole Armor (Ex?): Pasari-niml can tear holes in an opponent's armour. The opponent must be unable to move freely (e.g. trapped in a pasari-niml's pitfall or entangled). Creatures not wearing armor are unaffected by this special attack. Armor with hardness 15 or higher (e.g. mithral, adamantine) are immune to this ability.
Hole armor is a standard action with the same range as the pasari's melee attacks. If [something] the pasari-niml deals 1d6 points of damage to the opponent's armor and tears a hole in it. A single pasari-niml can then attack the vulnerable area at the hole, ignoring the opponent's armour bonus. Multiple uses of hole armor open separate holes, each of which can be exploited by a pasari-niml. Armor reduced to 0 hit points is destroyed. Damaged armor may be repaired with a successful Craft (armorsmithing) check.
I wondered whether it should be a Supernatural Attack, since tearing open armour seems a little extreme for a foot-tall Mant's mundane attacks, but I guess its no more unlikely than digging a Pitfall in seconds (indeed, the two abilities might be separate applications of one power).
That leaves the "how does it do it" bit. My first thought was a Strength check:
If the pasari niml succeeds at a Strength check (DC X plus opponent's armor bonus), the pasari-niml deals 1d6 points of damage to the opponent' s armor and tears a hole in it.
Or we could base it on HD:
If the pasari niml succeeds at a level check (DC X plus opponent's armor bonus), the pasari-niml deals 1d6 points of damage to the opponent' s armor and tears a hole in it.
Or the opponent might get a save.
The opponent must succeed at a DC12 Reflex save or the pasari-niml deals 1d6 points of damage to the opponent' s armor and tears a hole in it. The DC is Strength-based and include a +4 racial bonus.
What do you think?