It's not easy, but it can be done. My DD is/was in a toxic friendship and she needed help to get out of it. All those feelings and emotions that posters speak of on the relationships board about adult friendships where there are unequal power balances exist when the children are younger, and the more powerful child really is powerful in the eyes of the child with less power. That child needs outside help to pull them out of the friendship and teach them that it is ok to say no, it is ok to become unavailable, it is ok to create distance (physically and emotionally) and it is ok not say 'how high?' when commanded to jump.
Equally, we as adults can recognise that the powerful child will not be exercising power because they necessarily feel fantastic about themselves, and we can help our children to understand that it isn't likely to be anything about them, or who they are themselves that made them the chosen one, it just needed to be someone.
I explained to my child that this person wasn't a good friend for them, because they constantly made them feel bad and small and insignificant, etc. But, also, that my child wasn't a good friend for the other child, because that child needed a friend that would laugh at their attempts to game and control and say "Oh X, there you go again, get back in your box... Let's do this now." Someone confident and assertive would be a good match, and would help X overcome the insecurity that made them a bully. My child feeds that behaviour. No victim blaming, by the way, just observation.
I have found that having an understanding school is truly key (and rare). If they see the detriment of having certain children together, and can support those children in gaining wider peer groups, it allows for a more natural process of disengagement.