Lots of warriors usually helps.
Frostlings and wolf riders kill themselves trying to capture your cities. So will archers, if they do not choose to fortify themselves on top of some forrested hill, or a goblin fort.
Now if Mokka himself spawns near, You are in trouble. Your hope is numbers, fortify and let him do the first strike. If he takes some damage, You unleash everything You have on him and pray that is enough. If he survives and gets the experience You can pretty much pack it up and start a new game...
The thing is that Mokka and frostlings will attack Illians as well. So it's a hit and miss for the player summoning them.