So as I was searching the net for old fanfics of xellos and filia, I came across this AMAZING fanfic that had me up till 4am reading it. This story grasped the atmosphere that makes up the slayers series. Not to mention that their are tons of hillarious scenes as well as heart-wrenching ones. I don't want to spoil much but xellos his going through a monster's version of 'heat' called peak and the person he's set on making his mate is a certain dragon ^__^ I found it interesting that story centered on xellos being the one in heat, from my experience reading fanfics filia is the one cast in that role usually. Their is the pairing of syphiel/zelgadis that's in there but I kinda skimmed through it and read the xellos/filia scenes. It's definately worth the read! The story is set in a 3 part novel trilogy (when reading prepare for some loonng readin')
http://www.fanfiction.net/s/175295/1/Slayers_Faces The first story establishes the zel/syphiel storyline but their are many classic xellos scenes and reading this is neccesary in understanding the xellos/filia stuff in the next installment.
http://www.fanfiction.net/s/250296/1/Faces_After This where the the xellos/filia storyline starts and it's quite the ride ^__^
http://www.fanfiction.net/s/547522/1/Faces_Finale The last part of the trilogy.