Actually, thinking further about it, I wonder if the real problem is Lewis' (the author's) coyness about using names for God. It's obvious that Maleldil is Christ (or, let's say, the Son), but this is only ever said obliquely ("My name is also Ransome", or narrator-Lewis' reflection that "he knew who Ransom supposed Maleldil to be", etc). The one thing that is always part of an exorcism is an abjuration to the devil to depart in the name of Jesus Christ. But Lewis was clearly very reluctant to spell the identification out.

Still. It would have taken Ransome ten seconds to say "In the name of Maledli the Young, depart, foul fiend", and given that Ransome really isn't keen on unarmed signle combat to the death, you'd have thought he'd have given it a go...