Oh I got another one.
Final Fantasy 7 was my first real JRPG play-through and I was at Cosmo Canyon doing fine until that boss at the end of that place. You know, the undead guy. Obviously, the easiest way to beat him is to use some powerful healing item on it such as a Phoenix Down or an X-Potion. Things that you get moments before the encounter. However, being my first game like this, I did not realize that this tactic was a thing, so I tried to actually fight the guy. After something like 3 or 4 hours of dying, grinding, and repeating, I finally gave up and asked my brother to do it. I felt pretty dumb when he beat it in less than a minute then handed me back the controller.
EDIT: Should mention that my brother is kind of an ass and knew the whole time that I could beat it easily, but didn't tell me because my frustration was funny to him.