In that case, since theres no rulebook i can always add another element to the game to increase the skill required.
You're right in the sense that a player can't shake.
Off the top of my head, lets say every player is given a dragon spear and players can stun people when the npc isn't looking.
Then we can make the npc kill stunned players if they're spotted. The game instantly becomes much harder.
Lets say we also spawn a row of guards 10 squares away from the leading npc that follow us at all times that will kill you if they come into contact with you.
Now you can only operate with 10 squares of the main npc, all players can potentially stun and kill you and since you can only walk, falling behind elimates any ability to catch up.
I wouldn't count this as unrealistic when theres no limits to what i can do to it.