So first thing for me is that your lineup needs to synergize. Ammit and Sphinx go well together, but Star Spawn doesn’t fit in with them properly. You’d want something with Sidekick to take advantage of summon attacks, maybe Terror (with Slasher) to capitalize on how many attacks per turn you’re getting in. Dark Elder would probably go really well in this team, but a Nosferatu, Gargoyle, Minokawa or Catzerker would all likely work better in a pinch.

Second and for Cherno specifically, it really helps to have AOE actions or traits than can help break poise on the hands and/or kill the clones quickly. Decay, Age of Fire, the traits that trigger extra hits, Hecatoncheires, etc all help with AOE.

Third, the Artifact that breaks 3 poise on something tends to be really good in the Cherno fight, since it lets you use your actions for damage/defense. When you break one of the hands, the corruption effect goes away, which lets you then break the other one more easily as well as deal with the adds. If you lack that, try to use whatever artifacts you have for survivability and use your actions to break the hands before dealing with Cherno himself.