I have found that finding something, anything that a challenging alter contributes, and thanking them for it cuts through the animosity more consistently and more quickly than anything else. It may not work the first time or even first few times.
If you can only see damage from an alter, you can at least thank them for trying to help in their own way. You can be sure, 100%, that from their point of view they are trying to help. They most certainly were originally. And they are probably very alone and lonely.
When someone, an alter or an individual person, receives gratitude for what they think they are doing right, it can come as such as surprise they can't help but listen. When you get thanks or a compliment from someone, since you want that thanks to be for something genuine and you want the compliment to be true, there's at least some part of you that listens, that wants to validate that gratitude or observation.
The only detail is that your comment has to be genuine, to come from a genuine rather than a manipulative place. They will know. After all they share headspace with you!