I haven't beaten it on Intense yet, record so far is 5, but I just recently actually started trying Intense, before I was trying to build up the basic boss strategies.
I've found Ness to be pretty good at Boss Battles. All of his aerials work pretty well, fair, bair, and dair work the best in my experience, but uair and nair aren't bad either. PK Fire works okay every now and then, since it stays there for a while unlike Lucas's. His bat can knock off some damage quickly, his Up Smash can be used to hit aerial enemies, his PKT2 can also knock off decent damage. VERY important, however, is PSI Magnet, using it on the right attacks can replenish half+ of your life. For instance, Master Hand's gun attack replenishes 60%, I believe, and Duon's pink laser can heal massive damage.
So far, I've found those two to be the only ones that can heal you, I've found all of Meta Ridley's don't work, and Rayquaza's don't either except I haven't gotten a chance to test his Lightning.
From experience, I've found Rayquaza and Porky to be the most painful. Rayquaza is really fast, and he takes forever to take down. Porky doesn't take that long to take down, but his charging attack is really hard to doge on Intense since he's practically on steroids, and both of his lasers are a pain to dodge. Duon takes a while to bring down, but for some reason I don't find him as hard. Galleom can be pretty annoying, but most of the time I can dodge most of his attacks. Tabuu I haven't gotten to yet, and the other 5 I can pretty much take no damage on, although Meta Ridley's agility can be a real pain sometimes.
I tried the Samus method that someone suggested, since she's one of my better characters, and it's actually not too bad, but she just doesn't rack up enough damage. Also, now that I think about it, I should try R.O.B., since he's another one of my mains and his aerials are really good (especially his uair, for racking up damage).
Really, if I had to give one tip, it's hit and run. Get one or two attacks down and then back off for a second and prepare to dodge the next attack, it's the most useful tactic. Also, on a lot of bosses, even the non-aerial ones, I find doing aerial attacks to be the best thing, with an occasional ground attack. Mostly because it helps you to dodge any ground attacks and being in the air after you finish an attack tends to leave you less vulnerable than on the ground.