The Ivory Stalker is a variant of the Gray Stalker and an ANMC. It is much more powerful than other Stalkers, easily the most powerful in the game. What makes this boss so nasty is that it is fought directly after the Blizzard Chaser meaning that you may be low on supplies or HP/MP depending on your armor or how well you did against the Chaser.
This is also a good thing because the fight with the Blizzard Chaser allows access to Kyle's Handgun, the P229. This weapon effectively gives Aya 50 free Combat Lights which stun and cause the boss to become visible. With a loaded Grenade Launcher, this boss fight could be over in about 5 hits, provided there are a few grenades left after the Chaser battle.