I recently ran the Privateer Press Colossals Release Event for my local players. One of our players had a Mountain King too. So we played a 5 player, 35 point, battle group only free for all. My battle group consisted of eDoomy and two Mountain Kings. Between the ton of health they have, the ability regen quite a bit with the whelps and eDoomies spells it was a quite nasty battle group.
I really enjoyed playing with them both and it was fun watching peoples faces when I put them down. The only problem I really had was keeping eDoomy protected since I just had the two Gargantuans to try and keep stuff from getting to him.