I completed the game a few days ago. This final battle took me a few goes, but throughout the game i used to double-click to get Simon moving faster.

In the battle with Sordid, soon as my mouse cursor was available to select a spell i'd click on the required spell and then the item floating above, double-clicking to get him over there a little faster.

I don't recall making myself walk to under each item before casting, i cast the spell first and let him then walk over to it - not sure if that makes enough difference or not.

But aggressive clicking and prepping/casting spells before walking over to an item is all i can really suggest.

Hope you can beat it!