>>47663844Razor Leaf, Body Slam, Screens.
OR
Razor Leaf, Body Slam, Screen of Choice, Poison Powder (Toxic in Kanto)/Mud Slap/Synthesis (if you play without potions)/Other utility move of choice
You can also replace Body Slam with EQ that is available right before E4.
Meganium is limited offensively, but still has just enough tools to be genuinely FUN to use. And mono-grass is surprisingly interesting typing defensively. The few oh-so-scary-and-undefeatable matchups that Meganium has are very easily defeated with common Pokemon like Geodude or are pathetically weak like Falkner and Bugsy.
On the other hand i don't understand people having boner for Typhlosion - it evolves pretty much right before last Johto gym and before that its best moves are Fire Wheel, Headbutt, Rollout and Dig (no it does not get elemental punches till Typhlosion stage).