So I read through all of your comments and other answers. I think I have an idea for you to think about for your unique case, consider this:
Stay ahead of the calculation by pre-calculating the damage for the next interval. So right after damage is applied, calculate your next damage and presend it. So basically, send from the server to the client, 1) time for the next damage tick, and that next damage amount.
That is the general gist, of the idea. There is alot more to do with it, but if you understand that core idea, it can help with sync issues. I use a similar technique for keeping animation events timed from server to client in an another project.
Hope that helps. 