Boss Battles. Expert. Every boss from SSBB, at the highest level, practically all with OHKO attacks. Galleon. Rayquaza. Duon. FUCKING PETEY PIRHANA. And of course, tabuu. how we all hate you....
Yeah that was tough. The best characters for that challenge IMO are Fox (spam his FAir) and Charizard (spam his Side Special), those attacks should kill each boss in 5-6 shots. The main difference is that Zard can take more hits while Fox can dodge the attacks more easily, although its still really hard even with that.
Other hard things:
-Stage 6-2 in the NES Ninja Gaiden. Turns men into boys.
-The early game missions in Radiant Dawn. Everyone in the Dawn Brigade except for Edward, Nolan, and Sothe/Micaiah SUCKS. Especially when you have to take on the Laguz...ughghghghgh...
-Castlevania Portrait of Ruin LVL 1 Old Axe Armor Hard mode - Okay the final fight between Dracula and Death is just crazy. Your attacks do like nothing and they can pretty much kill you in one shot with any of their moves, and trying to dodge around their combined attacks continously gets really annoying, especially since your most effective attack - the throwing axes - is hindered by your small MP capacity, so the fight will draaaaaaaaaaaag on and on.
Also, your worst enemy in LVL 1 mode are bats. They deal enormous damage since you're only Level 1 and they are stupidly annoying to sidestep or hit.
-Purity Forest in Mystery Dungeon Blue - This game forced you to do this since it was the only way to recruit Celebi. You could only bring one guy, who was reset to Level 1 with none of his acquired moves or IQ skills, no items were allowed inside, you couldn't bring money, and you couldn't recruit extra teammates. The EXP required to level up as well as the stat gains attained also become VERY low after you're about 30 floors in (ITS 99 FLOORS OF PURE EVIL), so the enemies will catch up to you. And if you ever run into a Monster House, you're gone.
I did eventually beat it after many failed attempts with other Pokemon with a Kecleon. Yes, a Kecleon. Super hard to recruit, but those guys are the best Pokemon in Mystery Dungeon. Wide moveset, good level up movepool, impressive stats, and a decent level up rate. I managed to find a Pass Scarf and I just trudged right up there like a boss. The only problem was that Kecleon was absurdly hard to recruit. (You have to steal something from an in dungeon shop to get him to attack you - they then come at you en masse out of the woodwork and they will immediately hunt you down - they give little EXP for killing them and with the Friend Bow and a Level 90+ Pokemon (I used Articuno since it had good stats and could clear them out fast with Powder Snow and Mind Reader + Sheer Cold linked) they only have a .1% chance of recruitment.)
Other Pokemon I tried (Many times) were Charizard (starts with Heat Wave for multi-kills, got to Floor 56 before I was offed by a Tauros.), Electrode (Sonicboom was okay but he wasn't so hot...), Gardevoir (Teleport abuse, but I kept getting unlucky with items), and Mewtwo (fast Level Up with solid stats and a huge movepool, but he couldn't hack it against the Scizors and Houndooms that appear later in the dungeon, highest I got with him was like Floor 60?). The first time I tried Kecleon was the time I beat it lol.