There are not that many Psychic-types in Hoenn prior to National Dex but a good one would definitely be Gardevoir since it's also part-Fairy and can take down a fair good number of stuff plus it has some other additional coverage (such as Focus Blast, Hidden Power, Thunderbolt and Shadow Ball) and support moves. You can't get its Mega Stone until postgame but it's still a powerful special attacker on its own.
Gallade would have been an option but the major issue is that a Dawn Stone can't be obtained until Victory Road, so you'd be stuck babying a Kirlia and having to stick an Everstone on it until then, unless if you are good enough at Super Training but those mini-games can sometimes be very hard.
Gallade would have been an option but the major issue is that a Dawn Stone can't be obtained until Victory Road, so you'd be stuck babying a Kirlia and having to stick an Everstone on it until then, unless if you are good enough at Super Training but those mini-games can sometimes be very hard.