Ever wanted to bury your players under a rockslide and simultaneously start combat? Forget about goblins with levers, here's the avalancher!
With 6 magical beast HD, large size, and respectively +12 and +10 to strength and constitution, the avalancher would theoretically make a fearsome melee combatant. However, it is held back by a lack of limbs and only a single natural weapon (it's a 1d8 slam, in case you're wondering). Its other ability scores (ranging from 12 dexterity and wisdom to 5 intelligence) aren't anything to write home about either.
In terms of defenses, the avalancher has +8 natural armor and a very interesting immunity to bludgeoning damage. Total immunity to slams, tentacles, hammers, and the occasional unarmed strike is quite nice, if only I say so myself.
Mobility-wise, the avalancher has at least some stuff going for it. 20 ft. climb speed is easily forgettable, but the burrow speed has its niche uses.
Avalanchers have a couple of special abilities. 1/day Transmute Rock to Mud is a decent way to hinder grounded melee enemies, weaken obstacles of unworked stone, and dispose of petrified foes.
The other ability is Cause Avalanche, which does exactly what it sounds like. The rockslide created is moderately potent (2d6 bludgeoning damage, a chance to get buried with no clear answer on how to escape, and arguably a movement opportunity for the avalancher). The fact that it only works in terrain that is both sloped and covered in loose rubble ruins it, though.
All-around vision is hardly worth mentioning; the same goes for +4 on Hide in rocky terrain.
All things considered, I think -0 LA is appropriate here. The avalancher has one trick it can do sorta well, and which it can only perform in a very specific kind of place. As a melee brute, its secondary role, it is outclassed by a typical PC and has significantly worse growth potential to boot.