ray? naw~ the ledgends dun have hardkore advantage over normal wild pokes~ either that or i just happened to be fkn lucky with the ancient ledeges =S but back to lati..
this is going to be annoying but, first u gota actuali FIND lati first... it's hard if u've never actuali met it before, but if u have u could try running into it from mauville, since u get 4 diff routes from that city; if lati's nearby (like couple routes off) u get a chance to run into it if u go towards the route of that directin (chances are lati's headed yer way, but his flight patterns are random, this takes ages but it's faster than 'fate' lah)
... er oh to actuali CATCH lai ... (even more fkn annoying) gota use someone who spores or meanlooks... then they have to be faster than lati... the BEST way is to use wobbuffet, since lati can't run (and can't damage u as hard since yer psychic)... then the tricky bit is to catch lati asap before wob dies, i'd try encoring one of lati's recovery moves like refresh or protect n stuff like that n just keep chucking balls n hope u catch it... coz once u kill it, u gota look for it ALL over again -_-"
(to speed up the searching process, you could use the kanto trick with the dogs: your lead pokemon should be same level 40 and use max repel... then the only pokes u meet will either be lati@s, or some hardkore strong shyt that wasn't supposed to be there in d first place (level 41 tentacruel ?!?!?!)
good luck man, u'll want it ~ stock up heaps of repels n balls n pots (for wob) and off ya go~ u can either go hunt or just keep playing the game as pre usual trying to catch pokes (lati showed up while i was trying to catch an oddish near lillycove...XD)