Well, it's all about the boss behaviour in battle.
K'Tahu has several things to consider:
If there is a unit in his weak spot, there's 70% chance that he will move (change position) on the next turn (with the restriction that he can't move 2 turns in a row).
If there are 2+ units he can hit with his armored hand, he will do so with a high probability, rather than summon allies or spit.
If he can hit only 1 unit or he can't retaliate to a hit, he will either spit or summon allies. If there are already allies on the field, it is more likely that he'll spit, and if he spat the last turn, he is more likely to summon allies this turn, provided there are no 2+ units that he can hit.
Here are the chances (in general values, not percentage):
10 - to hit one enemy with his armored hand
110 - to hit two enemies with his armored hand
210 - to hit 3 enemies with his armored hand
20 - to hit an enemy in a side cell
200 - to hit 2 enemies in both side cells
50 - to do a mass attack (spit)
50-100 - Summon allies if there are less than 6 allies on the battlefield (including K'Tahu). The chance is 100 - Allied Troops*10 (meaning from 50 to 100)
When you want to minimize the casualties, simply work on these values. If you keep 2 tanker units near his armored hand, the chances are 110 vs 50 that he will attack them rather than do a mass attack, which is highly preferable.
It's the same vs. Baal.
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