I did it on Hard with a few more characters today.
I seriously think it's character/stage dependent. Most characters I have an easy time with. But even then, many of the stages can screw you over. My least favourites (in the context of All-Star) are Yoshi's Island Melee, Kalos Pokémon League, Woolly World, Port Down Aero Dive, Gamer, Castle Siege, Mario Kart Stadium, and Garden of Hope. Seriously, these stages are just so annoying to deal with.
My favourite stage is Norfair. The lava literally does 100% damage to the opponents in All Star (or something close to that), and it's so easy to make them touch it. Half of the time, the AI just walk into it. It can potentially KO them too. And they scramble around aimlessly a lot due to the platform layout.
Certain opponents are also kinda annoying. While he's not so much a problem, Dr. Mario is the most ANNOYING. All he ever does is use the damn cape to turn me around, and then other opponents body me from there. It's just a huge nuisance and looks incredibly stupid every time.
And some characters are just inherently crap at All-Star. It took me forever to do with Jigglypuff and Peach. They have no easy KO moves (Rest is not worth the trouble on Hard Mode), Jigglypuff has very short reach, Jigglypuff is SO light that the toned down launch power of your opponents is still threatening to her, Peach has an awkward recovery should you need to get back and can't really deal with three people bodying her at once, and Peach is kinda slow too and doesn't have the power to back it up for something like this.
I also had some trouble with Mega Man. His calculated and calm playstyle which works fine in normal matches, no longer works here. His attacks just have too much lag to deal with 3 people trying to body him. His Down Smash easily gets rid of people if you hit them with it, but you have quite the cooldown after using it.
Pac-Man was also a pain to do it with, but I think I was just burned out that time.