(My mistake, I thought it was Arnor's turn. I'll use my standard action to cast Enhanced Diplomacy on myself, then with the move action I'll try and talk Dimma down.)
With a faint, warm glow surrounding his holy symbol Calcius reaches out to Dimma, "Friend, you are outnumbered and outmatched. Lay down your weapons and I'm sure we can straighten all this out."
Diplomacy: 1d20 + 9 ⇒ (10) + 9 = 19