'completely mystified' is correct and usable in written English.
You can use it to describe someone who is feeling confused or bewildered by a situation. For example, "The professor was completely mystified by the student's answer."
The former PM and his wife are "completely mystified", the IoS can reveal.
"We're completely mystified," said Matteo Codignola, the Adelphi editor who translated the novel.
Perhaps he's unaware that large numbers of Americans remain completely mystified by the success of "Everybody Loves Raymond".
Ms Field, by now completely mystified, was accused of being indiscreet and in January 2006 Macpherson said she no longer wished to use her.
Victims are regularly left completely mystified as to how an incident that has caused so much suffering can end in such a paltry punishment.
"I am completely mystified and quite frankly disgusted that there has been a demonisation of this soldier," she said, saying it was wrong for Republicans to "make a judgment" about Bergdahl's service.
The Dutchmen are totally mystified.
Hakan's friends, colleagues and wife, Louise, all seem mystified about why he would vanish.
"She's hurt," I tried to explain to her, knowing it wasn't the worst thing that could have happened she'd been around her share of biters but upset and mystified, and frankly embarrassed, at how completely unprovoked it seemed.
All day, visitors and workers were mystified about when she might come.
On the surface, this is a simpler tale than "Have You Seen My New Blue Socks?" though young readers who still haven't completely distinguished their bears from their bees may be as mystified as Bear.
