I assure you that you will feel no pain at all.

我保证你根本不会觉得疼的.

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.
我们确信是在尽一切努力.

She's perfectly safe,I can assure you.
我可以向你保证,她绝对安全.

We assured him of our loyal support.
我们使他确信我们始终不渝的支持.