I think you would be better off treating the War Hulks as living siege engines rather than as elite troops. The class costs them all ranks in any Int or Wis based skill, after all. They are big, dumb, but STRONG things.
So, think along the lines of the Cave Troll in Moria in the Fellowship of the Ring movie. Give them handlers that get jerked around when the War Hulks smell blood.
As mentioned previously, how you deliver the description is everything. If there is something that your PCs have had experience with, like chopping through a door, show the War Hulks doing it easily.
Consider this as a staged scene ... the army laboriously rolls a battering ram up to the gates, and many soldiers swing it back, then let it fly. The gates shudder, but hold. Let the PCs lead a sortie that damages the ram and drives off the soldiers. Later, they hear the same >BOOM< >BOOM< sound, but more rapidly. There is a War Hulk using Power Attack on the gate, doing more damage than the ram at about twice the pace.
Or...
The advancing army comes to a small village where the PCs are stationed to observe them. The PCs watch the siege engines flatten a couple of houses... complete with vibrations felt a house or two away, and dust falling, etc. A passing War Hulk generates the same effect with what looked like an off-hand blow.
Hope that helps.