In the game betrayal at house on the hill. What are all the ways to stun the zombie lord?
I assume you're talking about Haunt 21, House of the Living Dead.
Rather than parse if &quot;can be harmed only by&quot; means the Zombie Lord can or can't be stunned, just look at the Survivor's Guide:

The attacks of heroes who are not carrying the Medallion have no effect on the Zombie Lord (they do not even stun it).

The base rules also say

Even if a haunt says to do something other than stun monsters when they take damage, they can still be stunned by effects that specifically stun monsters.

So, if there was an item that has the specific effect of stunning monsters that could stun the Zombie Lord if you attacked it without the Medallion. I haven't looked through all the items to find which, if any, that might be but I presume there are some since they included that clarification.
So if the hero posesses:

No special items - attacks have no effect on the Zombie Lord
Medallion - damage/kill the Zombie Lord
Items that specifically stuns monsters &amp; is not the Medallion - Zombie Lord is stunned.