Enemy AI units go mostly for:
1.) your most dangerous unit (your unit dealing type of damage AI is vulnerable to, or is strong shooter, or both)
2.) is weak against the type of damage AI is dealing (Dragons with Fire damage, will most likely attack your thorns as they will deliver much more damage/kills than if the would attack any other unit... +100% damage)
Also sometimes AI goes just for the nearest (this turn I can hit this summoned loser or noone... hmm I take the loser...)
The base logic I see behind AI logic is to deliver damage ASAP, not huge planning or strategy... so you can go for BAIT strategy using any spell or even w/o spells... AI is basically looking on type of resistance/damage and HP, it can't calculate well with it's own skills/abilities and not at all with yours...
So AI brainlessly uses up all its abilities and totally does not try to hinder you from using yours... the max we may await from AI is to try to hinder your shooters... but even this can be avoided by BAIT strategy
, so at the end, phantom or target... AI has many logic holes that players who are not first time in strategy world can reveal very fast and so sadly spoil the game little bit (not me, first I am not good stretegyst, that is also the reason for second point, which is : I love to rock'n'roll on bodies of my fallen enemies... )