Skallagrim said 'The rock is natural and not magic. Magic manipulates a natural substance. Since cutting things magic or otherwise from existence can be abused. You by virtue of your explanation cannot cut a natural substance, only the magic that manipulates it. As such it falls under a targeted hit and all rules pertaining to it.'

In reality it would come down to where the magic is coming from, and the answer is the circle on the ground. Cutting the rock would have no effect on the magic because it's only active at the bottom of the pillar. Also, that circle is pretty much the definition of prepped, so to escape unharmed with an unprepared defence would be a little dubious. 

There are a number of technicalities in this scenario which is why it would be best to have it judged, I don't want to interpret Skallagrim's words for him, and regardless of his personal situation at the moment which I completely understand is more important than this I still feel like he'd be a little irritated if we don't let him rule on what could be the most important moment in his entire tournament.