I also had a ton of trouble with the commander is the northern snow castle (the one that guards half the medallion for the secret not-DLC zones) too, if that's Niall. I totally forgot about him when I was listing the bosses that gave me grief. There really does seem to be a variance between players on which fights they find easy or hard, depending on their styles, so you're always gonna have "YMMV" situations.
Updated bosses that gave me grief: Malenia, Commander Niall, Fire Giant, Fallingstar Beast in Mines, Radahn, Mohg (final version in throne room), Hoarah Loux (the muscle-head guy, couldn't get near him, even though the first 70% of the fight with his other persona was easy for me)
Bosses that I had no trouble with, that other people seemed to: Maliketh (hell I've seen this thing listed at the top of "toughest boss" lists), Radagon, Rennala, and whatever that golden guy in the throne room was. Godwyn, maybe? The one who is normal-sized and casts spells. I rolled that fight, then kept reading about people struggling with it. And Godskin Duo. I think I won that fight on the first try, just immediately figured the pillars could be used to block them off.
Niall is probably the main one that I struggled with and haven't seen anyone else mention struggling with until now. And Radahn, he's either a joke or a wall to people.
My style of play is one that struggles with twitch-reflexes and instead looks for paths of less resistance. Like I'll identify a weakness or a cheese strat very quickly and down the boss with whatever I can, rather than measuring up to the speed and twitching required.