I agree with ringbearer. I think it would be more interesting if they had a completely anti-monster unit roster, since Dwarves can struggle against monsters.
I am not a fan of Dragonslayers, simply because they can double as a cheap Witch-King and have access to his level 10 ability by default so to speak.
I think it would be cooler if he had a single target attack only that does bonus damage against monsters, with the other Grey Mountain dwarves having some kind of interesting trait against monsters.
Like: Ered Mithrin Zealots, when being attacked by monsters gain knockback immunity, fearlessness and extra armour or speed and reduced damage against fire for 30 seconds or so, to signify their anti-monster role when fighting the dragons in the north.
The Spear Throwers should keep their ability and receive a buff to their stats or have something like their spears reduce the armour and movement speed of monsters by 20-25% by 10-15 seconds or so.
Finally, the Dragonslayer does high single target damage only with a bonus against monsters as I suggested.
I think it would give a niche, but distinct role to Ered Mithrin's units.
Any thoughts?