It took me about an hour to beat the boss, in total. Honestly, I didn't really start to make progress until I took it seriously. It really demands your full attention and skills, just like an old school boss...but it's super fair. I hadn't flexed this part of my Gamer Brain in such a long time that it took some getting used to, but it was well worth it. There's a consistent way to keep regenerating health and ammo, and once you learn it, the boss becomes much, much easier.
I also had like, 30% items or something when I faced the boss the first time. I decided to try beating the boss again with 100%, and yeah, it's infinitely easier if you have a zillion E tanks and missiles. I even beat it in one try the second go-around, haha. It feels like when I got really good at DKC2 as a kid and would just beat the K. Rool boss fight for fun. This game is incredible.