Hell no.
Nick has more experience in battle, he fought in WW II, Vietnam, Gulf War, he fought even alone against Yellow Claw, Hydra, and so.
He's an expert marksman, fireweapons user and hth combatant, the serum he received granted him to keep his body at peak physical condition and stopped his aging process or slowed him very much.
Nick has access to S.H.I.E.L.D. various weaponry, which includes hi-tech weapons far above those at disposal of Frank, and when it comes to hth, Nick knows how to kick your butt.
For reasons stated by K3VIL, Nick takes it, after a long fight though.
However, if they were let loose, given a Colt .45 with one clip of ammo, I think the fight would be a lot better than all assets. Still, I believe Nick would take it. He has years of experience over Castle, and the serum keeps him in top shape. Castle's no slouch, but his origin story is pretty solidly dated, meaning he's at least in his mid 50s now. Even a tank like him starts to slow down a bit.
Nick has been in every major conflagration since WWII, and most of the minor ones too. In the downtime, before SHIELD, he was a top CIA agent. Frank took all the training he could get and then some, and was forged into a warrior in Vietnam, and he has countless hours of fighting street crime. Nick has the edge in experience and fighting. However, they both know each other very well, and Ennis has transformed Castle into an infallible MacGuyver of Murder. It would be interesting, but I think Nick would pull it out in the end.
Gender: Male Location: Perfect night for hunting lowlife.
first its a one on one.
Frank doesn't only take on street punks, he has went h2h with some of the most skilled superheros around: spiderman, daredevil, wolverine, cap America just to name some off the top of my head. Can nick say he took on spidey, DD, and wolvie at the same time and come out on top, he is a brilliant tactician. All the s.h.e.i.l.d weponary, and serum wont save him from lead to the head. Franks just a better and more skilled killer in my opinion.
Gender: Male Location: Perfect night for hunting lowlife.
never said nick wasnt a skilled killer aswell but he hasnt spent 30 years fighting and killing criminals. Frank has killed a couple a thousand plus criminals and more than a few were no push overs themselves. Frank is the best one man army.