All the cakes that I have carved I have leveled, filled and stacked before carving. I pop the cake in the freezer for about an hour and carve from there. I have never had any problems doing it that way. Hope it goes well for you 
AYe I have buttercreamed mine first and the frozen and carved after all that, the other thing I have done is make two separate cakes the same size, freeze them as they are, then buttercream and sandwhich them frozen, let thaw for a few hours but so still slightly frozen and then carve. The frozen cake sets the buttercream well. It's so much easier to get a clean crisp carve when slightly frozen. I had to buttercream mine after freezing as I didn't have enough room in my freezer for a whole assmebled cake.
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