Ok for starters you can't use the golden hammer on this apparently. I don't know why you can't but whatever. Also the bosses seem to get harder the more you do. That aside, duon + rayquaza are the gayest to fight. I'm having a lot of difficult with both of them, using Charizard. The pirana is hard too, but I found spamming Razor Leaf to be alot safer, although it's really slow.
I havn't even gotten to Tabuu yet, despite me doing this for a very long time. One of the strategies I never see mentioned is... when he does his super rape thing you have to OMGDOGE.. just use Pokemon Change. It's long enough to completely avoid all the blasts. <.<
Anyway anyone have any good tips? Rayquaza and Duon give me the most trouble with their massive HP and gay attacks. I tried spamming Razor Leaf on porky but he's too fast and not as safe as it is with the pirana.