Or if you don't like weather, you could go
-Iron Tail/Reflect/Light Screen
I rarely use the screens (Reflect and Light Screen) except in serious battles like the E4, so I went for the weather setup one. The second set (if you take out the screens) would be a purely attacking Meganium, though you can throw in Synthesis in there somewhere... I just find it pointless to have Synthesis and Solarbeam if there's no sunlight active.
That's an all out attacking Heracross. You can also try Rest + Sleep Talk strategy if you feel like it.
Anyway. To make a long story short, I finally got a Lapras. ^_^ I trained Miltank to L26~ before going into the Burned Tower, then challenged Morty. Magby evolved into Magmar during one of the battles with the trainer. =D I debated for a bit whether to wait until she learns Flamethrower first or not, but in the end I just evolved it. It's only 4 levels anyway.
And after that, I went to the Union Cave. I had no Pokemon that can Surf, so I had to go catch Slowpoke first. >.> Anyway, now my team is complete, and I'm training Lapras to L30 to be on par with the other team members.
.. Speaking of training, I love that route where you can catch Miltank. In the morning, you can get Raticate and Pidgeotto for lots of EXP. And occasionally, Miltank and Tauros. Aha.
Aaaaaand I also have to start collecting money to get the Thunderbolt tutor for Lapras. Hmm. Wish I have an Amulet Coin now :x I wonder when I'll get that. *goes check a walkthrough* Apparently not until after Pryce.
Waaaaaait. *checks walkthrough again* Apparently the tutor is only available after the E4. Ah well. Hmm. Looks like I'll simply have to be content with ThunderPunch on Magmar for now.