In linguistics, we inventory the "sounds" of a given language; we call them "phonemes".
In Greek and Russian/Slavonic (as well as in Arabic and Hebrew), there is a sound that is made by raising the root of the tongue (the back part) so that it gets close to the back end of the soft part of the roof of the mouth (by the uvula - the dangling thing at the rear of the mouth). When air is blown through this space we get a "cccccchhhhh" sound, sort of like clearing the post-nasal drip area. This is the sound needed to produce the "ch" of PasCHa.
For non-Greek/Arabic/Hebrew/Slavonic speakers, the closest sound is the "k". If one's only language is English, then this is it. If one is willing to "hear" the 'original' sound, one should easily make the traditional "Pas-ccchhh-a" pronunciation. If not, no big deal. Do the "k" thing.
Blessings!
