Im sorry you and other people have an issue with this mechanic. I disagree with taking the mechanic out though. No mechanics should be taken out, we have too few interesting mechanics to deal with as it is. Every mechanic(even simple things like "dont stand in the fire") has some people who cant consistently get it. You can still see your groups HP totals after he uses Subtract, so its possible to try to copy what someone else does if you cant calculate on the fly. And the big hit after is survivable even if you failed to compute. Get topped off and throw down a shield and youll make it through(along with anyone else in your group who made a similar error).