In my first game I fought the twins first and it was insanely difficult but more importantly it just wasn't fun at all so I decided to skip the fights and try at a later date. Some weeks later I tried again with a more powerful V and more determination and the only way I could defeat them was trying over and over again and activating scanner mode slow-mo and jumping over their head and releasing slo mo long enough to try to punch them in the head. I did it with the guy in The Glen also and so I won those two matches. Tried Buck and the other one and they were too insane and not fun so I gave up.
In my second playthrough I dreaded the idea of the fights and avoided them for a long time, but then I was in a regular quest for a fixer to access someone's laptop or something and in the middle of the quest just randomly the game granted me completion of Beat on the Brat Coronado or whatever the eastern most one is. I was like wut, I didn't even go to any of the fights but the game completed the fight quest for me, a bug I presume or memory corruption or whatever. I had no problem with that though... Then a buddy of mine who had a similar difficulties with the fights kept at it until he finally managed to beat all of the fights, which motivated me to be more creative and I sunk the time in to trying to win them. I pretty much spent a long long time doing the scanner slow mo, jump over their head, punch them in the back of the head trick until I won, which felt like cheating or end running around a not fun game mechanic just to complete the missions. I ended up defeating the first 3 opponents after many hours, plus the freebie glitch the game granted me, and went to Pacifica to do the final match and it was impossible.
So... being a level 50 character completely maxed out in everything including gear fully modded, upgraded, etc. I decided to get Gorilla Arms and go around The Glen pounding Valentinos in alleys for a few days to get to level 20 Street Brawler. I'm at level 16 right now. When I hit level 20, going to do a few more upgrades from the new perks etc. and then go back for the final fight...
Wish me luck.
P.S. The upside of this is that I have avoided melee combat all along and it is immensely fun to go punching out gangsters all day without using a weapon and actually win. I can now go in and take out an area with up to 11 of them with just gorilla arms which is deeply satisfying LOL