To be honest, while strength training certainly can help be “harder” in the ring. Being “fighting strong” isn’t necessarily correlated with being “gym strong” at all. Same goes for any contact sport.
Some of the hardest hitting people, or those who play “hard” (in that when you contacted them they felts hard like a brick wall)in other contact sport like football or rugby were not “gym strong” at all… in fact many never trained.
I see being “fighting strong/hard” much like I see world class sprinting. A world class sprinter is born, not trained. Every body can get faster, but if you aren’t born to be a world class sprinter no amount of training can get you there.
I’ve trained a boxer who could punch through a brick wall (ok slight exaggeration 
) but who could hardly bench press 175lbs and a powerlifter who could bench press 500 raw who could hurt my little sister while punching.
Same with baseball pitching. Throwing hard (throwing a ball isn’t that different than throwing a punch) has very little to do with being “strong”. A good friend of mine is a pro baseball pitcher. He is also a crossfit gym owner. And he told me that he throws his hardest at the end of the season, when he is also his weakest.
I’m not saying that becoming super strong in the gym will not help. Provided that you do not lose your skill, mobility and agility, getting stronger and more powerful will always improve your performance. But if you want to “fight hard” you actually have to become a fighting machine by fighting all the time.
Also all those who seem to look like brick walls when they fight are those who are the most neurally efficient … they look like their muscles will jump out of their skins even at rest… they are always fidgety, always moving, talking fast and are very short tempered. They also all started to fight at a very young age and practice often.
I believe that “fighting hard” is more neurologicaly/psychological than it is physical.
I find that being very strong in movements like the deadlift, clean, snatch, zercher squat/deadlift, rows, pull-ups help more with wrestling and movements where you need slow strength than violent actions.
