Erm..."nature breeding combinations"? No such thing, me lad. Natures are totally random. HOWEVER, in Emerald you can influence them...
Mind you that the rest of this post concerns Emerald and only Emerald. If you have R/S, you'll be wasting your time if you read/do this.
Firstly, go scour your boxes for some Bold Pokemon that have the Synchronize ability or trade such a Pokemon to Emerald. If you need to catch one, you can try putting some Sour Pokeblocks in the feeders in the Safari Zone then catching several Natu/Xatu; soon enough you should get a Bold one.
Now that you have that, place this Bold Synchronizer in your lead and head over to the Fossil Maniac's house. Go all the way through and into the Underground Passageway where all the Whismur and Ditto lurk. With your Synchronizer in the lead, you should be finding some Bold Pokemon. Get a bunch of Ditto; one of them should be Bold.
Now that you have that, scour your PC for an Everstone (I think there is a minute chance of a wild Geodude holding one if you need one; get a Compoundeyes Pokemon to increase your chances and get some Covet/Thief user). Give the Ditto this Everstone and plop it in the Daycare Center with a Feebas (I'm not sure, but I think this only works with a female). Now you should be yielding Bold Feebas.
Enjoy
