for some reason KI moved most of the spells to other trainers(i have no clue why though) and they can be found in the stellarium, lunarium, floating land, stormriven, and stormriven hall.
As to why you cant use gargantuan on scarecrow is simple, only spells considered as damage spells can use them, like efreet, leviathan, snow angel, storm lord, frost giant and so on, scarecrow is a steal health spell so it takes the health and gives it to you so it would make sense that it cant use it on scarecrow