Depends on which outfit and which Sylv.
Edgy Maw “Everything needs to be destroyed” Sylv? I scream, grab the baseball bat I keep under the couch, and hit her with it. Then I run away from her.
BFA Tree burner Sylv: I start ranting about how she is a terrible person.
Sylv before they ruined her: S T A R E
These fantasies get stranger each time I see them on here.
Invite her in. Like I hate her as much as anyone, but a wooden door is not going to stop that woman if she wants something. Might as well try to be nice and hope it goes well. 
We need a thread like this for all the WoW characters, I wanna see the spectrum of reactions XD…
“Anduin is at your door”, “Helya is at your door”, “Sargeras is at your door”, etc…
If I answered I’ll close it.
If I look out a window & see her I ignore her.
Ion is at your door - Community / General Discussion - World of Warcraft Forums (blizzard.com)
Done.
I did one just for laughs…
Ask her to come back after she collects all her soul fragments and gets put back together while spraying Febreze and shutting the door mumbling about undead and odors.
live happily ever after killing people.for no reason:)
Give her a big hug and offer her a cup of tea 
spray her with febreeze
Ask her if she’s lost.
Empty a fire extinguisher in her face.
Introduce her to Ion who apparently is also at the door.
Ignore her.
Taps toes on the floor
I part the port and proffer her a double-aught buck facial.
" **
SylvANUS is at your door
** "
Fixed it for you 
Door is at your door what door do you door?
Ghostbuster the hell out of the banshee. She’s haunted Azeroth for far too long.
I will have a lot of explaining to do…
…to my wife.