First, note spelling Xmas. Or better yet, Christmas.
This is not an easy question.
"I met with her" seems to imply that you met for a a specific reason, that you had something to do or talk about together.
"I met up with her" is similar in meaning.
"I met her" could be a more casual encounter: You could also say
I ran into her.
I bumped into her.
if the meeting was entirely unplanned.
"I met" could sometimes imply that it was the first time.
Where did you meet your wife? I met her at a party in San Francisco.
I hope we'll meet some day.