the easiest legit ways to get them are trading and SR(soft-reset)ing
soft-reset is when you press A+B+Start+Select on the GBA and it will act like you switched off and switched on and it's a bit faster.
SRing for things like starters, castform, legendaries, beldum, etc is faster than going into an area and trying to encounter around 10000 pkmn.
there are no "special" ways to get shinies in emerald. There was a glitch to get a shiny ditto in RBY, and you could then trade that to GSC and then breed with it to get an increased chance of getting a shiny baby, but that increased chance (of getting a shiny when the parent were shiny) was removed in RS and everything after.
does this help?
(p.s. if you want to know the glitch in RBY, PM me)