I assure you that you will feel no pain at all.
assure:to tell sb that sth is definitely true or is definitely going to happen, especially when they have doubts about it 使确信；向…保证 ~ sb (of sth)
I know you think I did it deliberately, but I assure you (that) I did not.
我知道你认为我是故意干的,不过我向你保证不是故意的. VN (that)
We were assured that everything possible was being done.
She's perfectly safe,I can assure you.
We assured him of our loyal support.