What is the meaning and affix element for beneficial?

I think it is ial. The c before ial would NOT be included as part of the affix (also called a suffix in this case because it comes at the end of the Word) because the original Word is benefit (i think), and the t just gets changed to a c due to a linguistic rule for adding this kind of suffix in this kind of linguistic situation. This also happens when the suffix iary is added to create beneficiary. You can see therefore that the c is part of the original Word rather than part of either affix.

No. Its not ial. It is actually bene. Bene means good, well. the meaning for the root Word which is fic- the meaning is to make do. So the Word beneficial means- to have something do well.