So, I made a major pass on the AI for area-effect attacks. I had to anyway, since the addition of attenuation is kind of a big deal and needs to be accounted for by the AI.
A full discussion of how it works would be long-winded and boring, but in general the AI now considers the following factors when deciding when/where to use area-effect abilities:
1) How many 'valuable' targets am I hitting? (A target is valuable if the evaluation for that specific target is at least vaguely positive. It used to be 'if the evaluation is positive at all'.) Heroic valuable targets count as multiple targets.
2) How many targets can this ability's area reasonably include? (Based on the number of tiles the area includes. Roughly comes out to slightly less than the square root of the number of cells. Caps at 4, since that's the standard number of enemies in an encounter.)
3) How many enemies do I know about who are potential targets for this ability? (Ignores: enemies who are immune to the type, enemies who are way out of range of the ability, especially if they are not moving)
4) Is this ability unusually restrictive in its ability to affect targets? (If the ability is restrictive on any basis other than faction, this is considered to be true.)
Based on all of these factors, it ends up with two numbers: the number of valuable targets being hit by this particular ability/targetPosition combination, and the number of targets it expects it should be able to hit when using the ability.
If the valuableTarget count >= expectedTarget count, the AE is evaluated straight up (which means it will almost certainly be used, since doing a thing to multiple targets tends to be quite valuable!)
If the valuableTarget count is < expectedTarget, the evaluation is penalized based on how far short. Then, after the penalty is applied, if the evaluation has fallen below a certain threshold, it is set to 0. Note that an area-effect can definitely still be used in this case: it's just much less likely. This helps the AI save their SP/cooldowns for good applications of area-effects.
Will it fix the specific case you mentioned?
Well, I hope so! But, without being able to reproduce it exactly (I can certainly do "4 guys Chilled, one with Ice resistant"... but there's a whole lot of other factors in an AI evalution!) I can't tell for sure... we shall see.