What was the name of the demon who said to Lord Krishna that "I can finish the Mahabharata war just in second, but I need your Permissions"? & then Lord Krishna didn't allow him to do so.
I hope this might be what you are looking for. It is from Wikipedia. Better citation from a specific edition of the MHB would be appreciated But I could not find it Without a few weeks of deeper research. Hope this is of help to steer you in the direction of your choice. Blessings.
Barbarika alias Khatushyamji Baliyadev alias Shyam was a grandson of Bhima (second of the Pandava brothers), and the son of Ghatotkacha. Ghatotkacha was the son of Bhima and Hidimbi. Even in his childhood, Barbarika was a very brave warrior. He learnt the art of warfare from his mother. The gods (ashtadeva) gave him the three infallible arrows. Hence, Barbarika came to be known as "Bearer of Three Arrows". When Barbarika learnt that battle between the Pandavas and the Kauravas had become inevitable, he wanted to witness what was to be the Mahābhārata War. He promised his mother that if he felt the urge to participate in the battle, he would join the side which would be losing. He rode to the field on his Blue Horse equipped with his three arrows and bow.
Before the Mahabharata war began, Lord Krishna asked all the Pandavas how many days they would take to finish Mahabharata war alone. Bhishma answered that he would take 20 days to finish the war. Dronacharya replied that it would take him 25 days. When Karna was asked, he said he would take 24 days. Arjuna told Krishna it would take 28 days for him to complete the battle by himself. In this manner, Lord Krishna asked each warrior and received an answer.
Krishna disguised as a Brahmin, stopped Barbarika to examine his strength. When asked how many days he would take to finish the war alone, Barbarika answered that he could finish it in one minute. Krishna then asked Barbarika how he'd finish the great battle with just three arrows. Barbarika replied that a single arrow was enough to destroy all his opponents in the war, and it would then return to his quiver. He stated that, the first arrow is used to mark all the things that he wants to destroy. If he uses the second arrow, then the second arrow will mark all the things that he wants to save. On using the third arrow, it will destroy all the things that are not marked and then return to his quiver. In other words, with one arrow he can fix all his targets and with the other he can destroy them.