Pokemon Mystery Dungeon Explorers of Sky, Dusknoir boss and Primal Dialga boss, wow I was 10 years old when I battled Dusknoir it took me a few times until I have beaten him, and the Primal Dialga boss I was stuck on it for a loooooong time, I think for a few years because when I played it I was at the same part until one day I have beaten him when I was 12 or 13 years old, and I cried when I disappeared, but the Dialga battle music is epic. :D
Spectacularly losing to Golem at Cliffside Cave for probably the 10th time in a row. With a water type :>_<:
And when I finally do make it through, I headed back there again with a newly evolved Luxio and Kingler, and spectacularly lost again XD