To unlock access to this boss kill inside of the Iceberg, you will need to undertake the Mark of Spheniscomancy meta quest. You may have to quest request a few times to get the meta. You do not need the prowling orca phase specifically to unlock access to the orca.
Standard preparation. The fight should be fairly straightforward, and it's probably the easiest of the third class bosses. You can flee upward if needed.
Once you have the Mark of Spheniscomancy, you will always have access to the prowling orca, but the zone is level-restricted to 180 and under.