Honestly, not a boss but probably beating some of the tougher levels in SMW. Fucking Cheese Bridge, no small task for a 7-8 year old. Then, hmmm, bosses... Culex in SMRPG (for those who don't know, he was the secret, comparatively hard, "Final Fantasy-styled" extra boss in that game.) There was that "Wrong Side of the Tracks" mission in GTA SA, far harder than the "boss" battles in that game. Yeah, I dunno, I don't really gravitate to games with hard "Souls-like" boss fights.
Oh, you know what, the Wiz Pig car race in Diddy Kong Racing, or the second race against the octopus where he dropped bubbles instead of bombs and touching even one of them meant you had to restart.