Event 51 is a big challenge... but nowhere as hard as the REAL Giga Bowser fight.
Event 51's Giga Bowser is actually Bowser with Giga Bowser's special moves. I fought him using Bowser... and fount out that apart from a greater reach and from his access to Giga Bowser's insanely powerful special moves, the event 51 Giga Bowser does as much damage and can take as much damage as regular Bowser (and I'm not talking about Giant Bowser or Dark Bowser here; just the regular, plain playable Bowser).
Furthermore, Giga Bowser in Event 51 isn't nearly as fast and aggressive as his Adventure Mode's counterpart. He moves around slowly, abusing the Acid Breath more than anything else and often suiciding using the Earthquake Bomb (something Giga Bowser in Adventure Mode does not).
In short, Giga Bowser in Event 51 is little more than a moving shield for Ganondorf and Mewtwo and is, by himself, the weakest link.
So... if Giga Bowser in Event 51 is mostly lame... then why is the event so hard?
Answer: Mewtwo and Ganondorf.
People overlook them but they are the real, actual threat in the challenge. Mewtwo is a scaringly effective fighter that pelts the player with Shadow Balls from a distance and smack you with combos in close... and should you try to use his weakness (aerial moves), Ganondorf will Shadow Dive you to oblivion.
Giga Bowser comes off as a serious threat because that your attacks does not hit Mewtwo and Ganondorf as long as he lives, the big bad Giga Bowser serving as a sponge while the two other villains get free shots at you.
This why Jigglypuff comes in handy. Her Rest attack is one of the game's strongest moves and since Giga Bowser is so huge and does not block and evade attacks, unlike his Adventure Mode counterpart, you can easily knock him off the platform. Once this is done, you still have to defeat two rock-hard CPU opponents... but as least, you can now actually strike them.
In short, Event 51 is quite hard despite the event's version of Giga Bowser being a big joke, in truth...
I beaten that event with Jigglypuff (DUH!), Bowser, Roy, Pichu, Mewtwo, and Samus. The hardest was with Samus ; her Charged Shot is equally strong compared to Mewtwo's Shadow Ball but leave her vulnerable while charging and most of all, Mewtwo has Disable (VERY HANDY to keep that pesky Ganondorf from Shadow Diving/Wizard Footing you to oblivion).
Jigglypuff and Roy were uber easy (Roy's Flare Blade is a great Giga Bowser finisher and I'm skilled enough with him to defeat the Mewtwo/Ganondorf duo).