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I can't really see how kroak would be worth it for his points I am doing a very very heavy 3k magic list but a 2nd gen slann is almost as good and a hell of a lot cheaper. why use kroak
I've struggled to get the best out of Kroak in the past (and I'm generally regarded as one of the best users of magic in my 2 home GWs). For such a huuuuge popint sink he needs another Slann to back him up, and a few skinks running around, and suddenly, pop goes most of your points with no fightability on the board yet. He does well in seiges (destroying entire wall sections can be funny), but a couple of bad spell rolls and you can be stuck with one of the most powerful mages in the game having a load of not great spells. A very big gamble.
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totally agree with your question. If you can get a 'superslann' (as a like to call my 2nd gen slann with 2 plaques) for half the points, your paying twice as much for one extra wound, some fancy close combat stuff, and the ruination of cities.
But then, if your slann gets into close combat your tactics have failed.
Can't say i've actually tried ruination of cities, but it cant be that good? think of all the cool stuff you can get with the leftover points.