The word "completely" is correct and usable in written English. You can use it to emphasize the extent to which something is true or to emphasize that something is true in every way. For example, “I am completely satisfied with the outcome of this project.”.
I now regret it completely.
"Mo's training was completely chaotic before Alberto took over.
I understood completely.
"I'm completely lost.
I was completely wrong.
Catherine West, who beat Lynne Featherstone in the Red Wedding of the Lib Dem's election, creates a completely different atmosphere, but it's also one of infinite possibility.
It got worse: "There were two men who were particularly rude and completely dismissive of what I did".
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union