Firstly, you can start raising a Tympole for your future Seiesmitoad, and use that as your third Pokemon (Found Pinwheel Forest Entrance). Try to evolve it before Castalia Gym, Leavanny will knock it out.
Secondly, a guy in the Pinwheel Fotest entrance will give you TM 94 Rock Smash. Teach this to a Timburr (Found once again in Pinweeel Entrance) and strike at Lenora's weakness. After you have beaten Lenora, catch a Sandile for future Krookoodile on Route 4. As your 5he pokemon for castelia Gym, raise a Darumaka rrom route 4 and use him to help the battle.
The best place to train yout Pokemon before Lenora is deep in Pinwheel Forest (Bring Escape Ropes). Audino are EXELENT for traing.
Just Before Burgh, Desert Resort is great, especially Relic Castle. Get a fair idea of how far you can go while in there, youll need the info later in the game.
To beat Lenora, my reccomendation on your teams level is 23+, and for Burgh, 25+.