First the Server should be doing the Hits that calc dmg. Clients should never send a value to apply as dmg. There’s nothing stopping a malicious client from saying it did 10,000 dmg.
Second the server should be doing the Math and Setting a replicated var. That value will get replicated to every client. You do not need to have every client do the math. This is the whole point of replicated vars.
Alternatively you can set the HP variable as Rep Notify which will execute its OnRep Function. That function can be made to virtually anything on the client.