I'm thinking 'honestly' but I'm not sure that it's captures the harshness or criticalness of the situation. I want to describe someone who is not speaking overly critical of someone but exactly critical.
How about, bluntly?
To speak bluntly is to 'speak without trying to be polite or considering other people's feelings'
You can use the adverb frankly especially when the harsh truth you are speaking is from your emotive opinion.
"Frankly, good lady, I find you repulsive!"
John decided to be frank with Sam and told him that wearing a Hawaiian shirt to this occasion was not in good taste.