To add punctuation for (hopefully) better understanding.
The most important point for a Japanese learner would be to notice that a whole verb phrase is left unmentioned at the end of the sentence. From the way you worded your question, I could not tell if you noticed it.
It is important because that is the main verb of the sentence.
"One does something", "Something happens", etc. in just a second (一瞬のうちに). What that thing is is "omitted".
No translation attempt would be good enough without correctly "guessing" what the omitted part would be saying. In your case, you actually mentioned the verb phrase around the middle part of your question. It is "to kill the other guy". That is what will happen in just a second.
"Surely, if I used Hierophant Green, I could kill him in a split second without even giving him any time to feel pain."
「も」 means "even" here, which I used in my TL.
The double 「を」 are necessary. "feel pain" and "use H. Green".
「あたえず」 is negative. "without giving him ~~"