on 7 dice it is possible to get the 13th without IF, but not without 6's, so becalming cogitation is the way to counter it. also if a seer has it (and probably all will for the psychological effect) its a good ideea to avoid miscasts, a dead EotG/Slann is worse than a whole saurus/TG unit turned into clanrats so even the cupped hands become a high risk item.
Do the Skaven no longer have the irrisistable force special rule where if the sum of the dice is 13, its IF, not if there are double 6's? In 6th ed double 6's did nothing for them, I didn't know it had changed back.
Hmm well that makes the spell quite impractical really. As mentioned, you would basically HAVE to get IF to cast it. There is the same chance on the dice of getting miscast. However, if you roll both then the miscast happens so there is a greater chance of miscast than IF.
The other way of getting it off is the Slann or skink priests miscast and roll 5-6. OR, you're outside the range of bacalming cogitation. Besides, seers generally aren't going to cast the 13th on THEIR turn anyway, since they generally have more effective spells to spend those dice on.
depends on the player really. if he tryed to get with for the grey seer he migh have 2-3 plague spells with an ingredibly short range (biggest range is plague at 18"). also its quite impossible to be outside becalming cogiation range since that has a 24" range and so does teh 13th. besides the fact that the grey seer is usually on a bell in the center of the army any skaven will probably try to stay within range of the slann for that time when you miscast, there are no beter targets when playing against lizardmen, and with a few demons/vampire bunkers a slann+TG unit is probably the best target you can find in the game