Ruling on Irresistable Force

I need a ruling on irresistable force before I play as the Lizardmen again.

I have come to the realization that the question is only relevant for maybe two armies. There are no generic spells that cost more than 12 base, so if you roll irresistable force (2x6=12) you have met the casting value of the spell. So the argument is almost pointless. However, there is at least one army with a spell that require at least a 15 to cast. There may be some with a 13+ requirment, but that is irrelevant as well because you would have to roll 3 dice, meaning you would meet the value if you rolled IF ((2x6)+1=13).

However, it will affect the way I play the Lizardmen because of Lord Kroak, so I need a ruling.