These events are only playable with two players. You cannot team up with a computer player in this mode. There are no Challenges tied to the multiplayer events.
A Lurking Menace (Within 15 seconds) The 15 second cutoff point is the point where a giant Bowser appears. He's not too tough if your only goal is to clear the match, but you'll need teamwork to get rid of Bowser Jr. before that happens. Try knocking him up onto the top platform and finishing him off with an upward smash attack or Luigi's up special.
Wrecking Bros. (Hard) Your goal is to destroy the building on the Wrecking Crew stage. This actually isn't too tough, even on Hard. Focus on the bombs, and ignore the ones that aren't near destructible walls. Those are red herrings. You can easily destroy a bomb with a quick side smash attack. Hammers spawn frequently and they can help you out, but remember that sometimes their heads can fly off, making them useless. Don't worry about Wario and Dedede. Avoid their attacks but there's really no need to KO them. Knock them away to give yourself some breathing room though.
A Royal Errand (Normal or higher within 80 seconds) You have just over a minute and 30 seconds to clear this level by getting 500G between you and your partner, even without meeting the optional goal. Hit 'em hard to knock out as many coins as possible. They'll also drop coins if you KO them. If one of you gets knocked out, however, you'll fail the event immediately.
Food Fight (Normal or higher within 60 seconds) This is the Great Cave Offensive level and you and your partner start at 100% on Normal, meaning you're doomed if you even lightly brush up against a danger zone. You have a bit more wiggle room on Easy, starting at 80%. Your opponents will seek out food to heal their wounds, though this won't help them much. Picking up even one tiny item will put you below the instant death threshold, though, so go for it. You're on the verge of death, but a cornered animal is the most dangerous. Play this stage basically normally.
Viral Visitors (Within 60 seconds) You'll have to fight a total of four Kirbys as Dr. Mario and Nurse Peach. They come two at a time. That's it. There's no trick to it. Just KO them as fast as you can. They're light.
Unlikely Allies (Normal or higher within 80 seconds) This seems like a cakewalk at first. 16 Game & Watches, and look how tiny they are. Then you'll meet the two large ones, and they'll be a bit tougher, but take a bit more time. Then you'll meet the last two, who are huge, and metal, and go "Oh, come ON!" This is a walk-off stage, though, so just lure them to the edge of the stage and toss or, if you're Ganondorf, smack them off. Even the last few can be taken down quickly that way. Don't bother with that strategy for the tiny ones. They're nothing to you.
Flat Fracas (Normal or higher within 50 seconds) This is actually pretty easy. The giant Donkey Kong can be KO'd instantly by luring him to the edge of the stage and just grabbing and tossing him out. Mario will respawn after you defeat him, but you can deal with him either the normal way or the cheap way.
Pokemon Multi Battle! (Within 40 seconds) Your opponents actually have four Pokemon, who will attack you two at a time. This means you basically have 10 seconds to KO each Pokemon. Luckily for you, they're easier than normal to knock back, and one of you is Charizard. Try and corral them together so you can both damage them both at the same time, and have Charizard finish them off.
Getting Healthy (Within 40 seconds) This is a 150 HP stamina match against Kirby and Pac-Man, two notorious gluttons. Little Mac can probably take one of these guys down within the time limit pretty easily, but Wii Fit Trainer might have a bit more trouble doing things the normal way. This is yet another stage where you can just drag someone to the edge and KO them immediately, and that works in stamina matches, too, so if you're having trouble doing things the normal way, try it that way.
1988 (Within 60 seconds) You'll have to fight three Marios here. The third one is big. The walk-off stage strategy is possible here, but tricky, because the stage is huge and it only works in one specific spot. Instead, lure them up to the top layer and KO them upwards. Especially from the platform where Pauline normally sits, and ESPECIALLY the big Mario, because he'll take a lot of punishment otherwise.
Full Speed Ahead (Normal or higher within 25 seconds) That doesn't sound like much of a time limit, but the battle moves at a higher speed than normal. It's tough to control because of this, especially for Sonic. If you can use the racing machines in the first landing spot against your foes, great. Otherwise, there's not really any trick to this one. Just do your best.
A Fair Night Trip (After defeating at least 20 enemies) It's raining Kirbys in the Pac-Land stage. Your main goal is simply to survive until the point where you go through the door and meet the Fairy Queen. The Kirbys are light so taking down 20 of them shouldn't be a big deal. In scrolling stages, it's easier to KO enemies by knocking them "back" (to the left in this case) instead of forward/right.
Scheming Sorcerer (Normal or higher within 90 seconds) This is tricky. You'll start off fighting one Robin, but when Kamek shows up at about 45 seconds to switch the stage to the tower version, a large, metal Robin will appear. He's very difficult to KO, but he's also really heavy. Knocking out either Robin will cause them to be replaced with a female version. You'll have to beat all four to win. If you can weaken a large one and pop it into an egg as Yoshi, you might be able to get it to drop off the edge long enough to make recovering difficult for its large, metal body. A beanstalk will always grow in the first phase of the level, meaning it's easy to lure your foe up close to the top KO line of the stage.
Poisonous Planet (Normal or higher) A 200HP stamina match where all four participants are constantly having their health drained. The two Charizards hit hard, and you'll have to act quick to make sure they're both dead before you are. Basically, if you can keep their HP lower than yours, they'll die first. It's possible to KO one or both of them by knocking them out of the stage, but that's easier said than done. In this case, it's better for each player to focus on one enemy, rather than ganging up on one of them.
Secret Smash (Hard) If either of you gets knocked out, you'll fail. Both you and your partner absolutely have to survive this entire event. Also, your two foes each have two lives each. That's hardly fair! This is the Game & Wario stage, so mom will pop in from time to time to freak out and KO whoever she sees. This means you'll lose if you get seen even once, but you can also use it to your advantage if you can knock an opponent into her field if vision while you stay hidden.
Visiting Onett (Normal or higher within 60 seconds) This stage is pretty simple for a 2 vs. 6 team battle. Lure as many as you can over to one edge of the screen and just smash them out of bounds. Watch out for the cars, of course. They can be a help or a hindrance, depending on your ability to dodge them.
Mirror Magic (Normal or higher within 30 seconds) These giant metal characters won't go down without a fight, but they don't actually have to go down at all. They're huge and in the way, but Ganondorf is your real target. This stage starts in the second phase of the Fire Emblem Castle Siege stage, so you can actually just drag Ganondorf over to the edge and toss him out instantly. The giants will make that more difficult than usual, but not impossible.
Solidarity (Normal or higher within 90 seconds) You'll have to beat Bowser and his son after you beat Mario and Luigi. There's no secret here. It's a simple 2 vs. 2 match followed by another 2 vs. 2 match.
Peach in Peril (Normal or higher within 50 seconds) Ugh, that idiot. You'll have to fight five female characters while trying not to accidentally KO Peach, who is just wandering around the stage uselessly. She's vulnerable to your attacks, meaning Bowser's ability to clear crowds with big, powerful attacks is a huge liability. At first, it's simple enough to keep an eye on her and try to focus on enemies who are on whatever side of the stage she is not, but once you cull the herd a bit, they have a tendency to just bunch together. Your grab is your most precise attack, so use that to carefully pick your enemy out of the crowd and toss her off to the side to separate her from Peach.
An Offering of Coins (Normal or higher within 60 seconds) You'll need to collect a total of 1000G to clear this coin battle. Heavy attacks bring out more coins, but Pit doesn't have many of those. Their side specials will do the trick, as will their side smashes. Absolutely do not get KO'd, because you'll fail the event instantly if either if you goes down. Both of you have to survive the entire event.
Final Battle Team-Up (Hard) You'll have to fight seven villains, including a couple who are just palette swaps of heroes. The most noteworthy thing about this match is that you can choose your character. You might have an easier time clearing this event as someone who hits hard and is difficult to knock away, like Bowser or King Dedede. It's actually pretty straightforward. There's no trick to it.
Keep 'Em off the Ship! (Hard) Just like the single player event, play as Fox and Falco and keep the weak Game & Watches from landing on the ship. Some will float down on parachutes and some will just fall. You can see where they'll be coming from by keeping an eye on the magnifying glasses at the top of the screen. Upward aerial attacks and up specials will get the job done. The easiest way to do this with teamwork is to have each player cover one side of the ship. This will make sure there's someone free if an enemy suddenly appears on the other side of the stage.
Sky Pirates (Normal or higher within 50 seconds) Your reward time limit isn't that much different from your regular time limit. Neither Meta Knight nor Dark Pit are known for their great finishers, but you'll have an easier time if you use Dark Pit to finish them off while Meta Knight focuses on damaging them. There's no trick here. Just do your best.
Robots vs. Dragons (Within 70 seconds) When Ridley appears, you might think "Oh! Good, I'll just knock him down and let him do the work!" Then you might knock him down, and see that he hasn't joined your side, and say "Oh, dang, guess I'll just have to focus on the enemies." After a bit of struggle, you'll KO them, and notice that the level isn't over, and realize that Ridley is a dragon. Yes, you'll have to KO Ridley. He goes down after taking enough damage, not after being knocked out of the ring. Sometimes he'll leave the ring and be out of reach, so it's probably a good idea to focus on him whenever you can, because the other guys are always vulnerable.
The Ultimate Battle (Hard) Fight every single character in the game! That sounds more intimidating than it is, since they're actually very weak versions of themselves. A big brute like Bowser can KO them with one hit, so that's what you should probably do. This is actually a surprisingly easy fight for a final battle.