Anyway, I completed the basic attacking code on the server-side. Player-characters can attack monsters and then the clients receives damage updates when the target is hit. After a predetermined number of hits the monster dies, the attacking stops, and the client receives a "kill" message followed by a "remove" message.
There is no check for range yet, so players can attack anything on the map no matter where they are.
The next step will be to implement client-sided support for combat:
- allow to select a target for attacking by clicking
- visualize damage