Tank in Leviathan EX

[HumsterMKX]

As far as I know they are stunable. But there is also a high chance it got resisted or, the same cast bar problem we face. Aka, Titan plumes issue, the cast bar is not the same on server and client side.

[Atreides]

Like I already mentioned, all his skills are stunnable.
The add just happens to build a stun resistence on his own, it doesn't matter whether someone else was stunning it or not more often then not it starts resisting because you took too much time to start the stun chain while many offtanks seems to think someone else did it.
If you keep stunning it it doesn't build the resistence over time so the best way is to just stunlock it after when it hits 80% or like 5 seconds after it spawns.
When I played monk I was sometimes accused of stunning the add because I was the only other person able to do so, they shut up when I charged back to leviathan and Steel peaked it as I explained from my own experience as offtank how the mechanic worked.
Thats why I hate going with randoms~ D:

[Avalon_Albrook]

Like I already mentioned, all his skills are stunnable.

And as has already been mentioned, all his skills are not stunnable. Only dreadwash is stunnable mid-cast. You cannot stun dreadstorm mid-cast. Stunlocking an add to keep it from getting the chance to get a cast off is not the same as that cast being stunnable.