So i've been working on a multiplayer game for a bit; it's a co-op action RPG with real-time combat. If you've seen or played TERA, I'd say it's comparable to that, but not an MMO, heh. I'm currently handling the AI units authoritatively, the server calculates their pathing, movement, and pursue/attack logic, and syncs the movement to the clients 15x per second, and the state changes when they happen.
When I emulate 200ms ping, though, the client can perceive being out of range to an AI's attack, but still take the hit, because on the server he hadn't moved that far yet. This also plays hell with my real-time blocking.
I don't really want to allow the clients to be allowed to say "that was out of range" or "I blocked that", but I'm not really sure how else to handle it.