If we were going by numbers I would say ISFJs no contest.
But since you said individuals...
It would be close as neither one would really probably want to fight.
I would suppose it might come down to what they were fighting over. If the ISFJ was trying to protect someone close to them, or protect something they held dear, I could see them pretty much going feral. Putting their all into something off of pure adrenaline, being prepared to go down with the ship. If they are really angry an ISFJ might legitimately destroy you. The powers of angry Fe can be dangerous, they would be able to twist whatever you say if it is a verbal battle. Si, considering this is a long term scheme to tear you to pieces, might be able to synthesize a great plan, based on what they know, to tear you apart. So immediate heartfelt adrenaline attack followed by measured strategy to pretty much rip apart the basis of your character. An ISFJ 6 would probably be best suited.
Then again. If the INFP was in a fight for something that they really cared about, their line of defense might be longer and less emotionally raw than the ISFJ's. They might end up having an easier time with understanding the basis of what the other person is doing and might be able to shut it down based on that alone. Might actually be more commited to their view and harder to convince or shake than the ISFJ. While the ISFJis using social manipulation to undermine the INFP, the INFP might be able to subtly manuever out from under it-negate the ISFJs whole argument by really understanding the entirety of it and counterstriking with an irrefutable point, and come out the victor. An INFP 5 would probably have the greatest advantage at a verbal spar.
So actually it would depend of the extent of either whose values are crushed. Could go either way. Base adrenaline would help an ISFJ, quick thinking an INFP.
A physical fight might be too close to call. Whoever gets the upper hand first.