Christina is a very overrated singer who strains often and sings off pitch quite a bit live, specially in recent years, her real vocal range where she doesn't sound like she is in physical pain is very narrow, so for her it's best to:

1- Sing easy listening or smooth soul kind of songs where she can sing soft and smooth, quiet storm type of thing
2- Sing in a narrower vocal range of 1 octave or a little more but not trying to stretch it too much or her low notes are foggy and unsupported and her upper notes are strained and tense.