I tried this for the first time last night and just got completely frustrated. Fox is definitely the way to go, but DK is surprisingly quite a beast if you can get him down. I prefer Fox though because he has a TON of ways to beat the toughest bosses.
All of them are just all about learning how to watch their attacks. Use fox's UpAir and Fair and then run away accordingly. You'll have to learn the timing on your own, and usually it takes practice. Along with learning how to air dodge.
The ONLY bosses you should have trouble with after learning all their timing are Duon, Galleom, Porky, and Rayquaza. Tabuu...with him its just a matter of learning his attacks and timing them. I found him to be more of a relief. Hardly any of his moves are one hit KOs. And the ones that are...are really really easy to avoid. Anyway, for Duon...pray you fight him early. VERY early. like within the first 3. Because I noticed that the bosses you fight later (6-10) no matter who they are....are signficantly faster and stronger. Duon's usually beastly attacks only hit for 15% or so if he's the first one you face. So if he's your biggest problem, just repeatedly restart until you face him first. That can be annoying though, so I just play no matter the difficulty.
For Duon though, I highly recommend shooting a few lasers to stall for the first attack. The moment it says go. Its going to just run in and attack you right off the bat. so its best to be prepared. Ironically I found that you can run to the edge and edgegrab. then fall and firefox back up grabbing the edge and repeat. Do this until Duon goes back to the other side. Then wait for an attack. and if its not a rushing across the stage one. run like hell and try to get a few aerials in. then run back. and wait. repeat this until it switches to the pink side. this is your chances to win basically. attack like crazy and when it does its lasers, reflect them. Make sure you are close because then you give more damage. The lightning thing? just try to dodge. I usually run away to the other side cuz it gives me more spacing and time to avoid it. But don't waste too much time trying to "play it safe" fact is, this thing is a beast and will rack up damage. And its incredibly hard to avoid its dash attack because if you jump and air dodge when it is finishing up it hits you straight up when it rears up on its edge to stop. yah...even this kills you. So when it changes to pink, rack up as much damage as possible. PRAY that it shoots out the missles cuz if you've been racking up damage and continue to do while the missiles come out,then you have a sure fire win. when you see them come out, simply run away for a tiny bit. Then come running back and attack. usually you will take a blow from one of them but the rest hit duon dealing an incredible amount of damage. Usually this is where I win.
Galleom, This guy is just scary fast if you face him later on. He's almost TOO fast. the missles are very hard to avoid, except if you run off stage and into the magnifying glass. Which is what i recommend. No joke. You will slowly take 4-5% but its worth it in comparison to taking a risky 15-30% or if you already at high damage, death. then come back. Constantly fire lasers when you can, because Galleon takes a decent amount of damage from these for some reason. when he does hes shoot across the screen thing after laying down, jump and air dodge. then run back as fast as you can to where he will end up. then up smash and run away quick. You absolutely must avoid the shockwave move. The one with the thing that travels on the ground. This can do anywhere from 60-120% damage if your not careful...and yah it kills you almost every time if you already have some damage on you. Don't let him fall on you, but be just out of his range that way you can get in a quick up smash. This battle usually takes me awhile and I'm either doing really well or really struggling. So I think as you practice someone can absolutely bone him.
Porky may seem tricky at first, but is actually quite easy once you get the strategy down. first things first. Only start the match by shooting lasers. They don't do much, but any damage is better than no damage. Porky only has two ways to start the match. The super fast one hit ko moving fast to the left side of the screen move(yah thats what its officially called...LOL) or attacking while moving towards you no matter where you are. If its the first its obvious. Prepare yourself. Jump over and dodge. then start running back to where you know porky will end up. the second its back. Up smash, down smash, and side smash. then run away and see what porky is going to do. If porky does the laser move try run away to the far right as much as you can, but make sure you can see yourself. make sure you jump, dodge the first one and then keep holding R and fast fall when the laser comes back around to hit you the second time you should be shielding, don't hold it any longer then when you hear the clank because your shield will break, and trust me...you will be pissed off when porky decides to do his one hit ko move one more time especially when your super close to the blast off zone. Now if it decides to do its walking slowly towards you think. Jump on top of it. and down smash. if you can keep your momentum and stay on(which is hard you can have an easy way with porky and avoid 2 of his moves(the down laser beam[not the top one i first mentioned] and the porky kids that chase after you and explode). If you cant though just keep trying to do it while doing forward aerials. If he does his repeated hit side thing. its like a little spike that hut racks up 60% if you get caught in all of it...and it usually kills you in one hit too cuz of the knock back of the last one. You can DI out of this move really easily if you do get caught. Make sure you DI down, not up, if you DI up you will fall back into it and die. If you can DI(I use smash DI) down then shield and roll out if its range. then fox illusion to the other side where its safe. If its going in its ealry stages you have an opporutnity to get about 3 smash attacks off. So this is a good way to rack up damage. Use fox illusion if you see that moving coming before hand.
And as for Rayquaza...reflect his attacks back at him that you can(i dont think i need to name them or mention them) Make sure you avoid his tail hit move and his glaring eyes move. Those are KO moves along with his sidewinder one across the screen. that one is easy though if you see it coming. Just jump twice and airdodge. you must time it perfectly though or you will die. Rayquaza seems nasty, but once you get his timing down he's not TOO bad. However if he's fast from the get go you are probably screwed cuz he's so intense with his attacks. laser's don't work too well. I always do em if its better than nothing, but you have to run in and attack. But if Rayquaza seems EXTRA fast. Only go for one smash at a time. This is lame, but not so risky. And if you think Rayquaza is doing his tail attack thing on the ground. Fox illusion to the side to get out of there if you are too much to the left.