That's not really going to help it against Morty, Jasmine, Team Rocket as a whole, and Silver, who all carry Poison and/or Steel types (who can't be poisoned) and also usually have Pokémon that either outspeed Meganium or can cripple it in return. Some like Clair and Pryce also give it a hard time even if you do try stalling since their Pokémon resist its typing, attack on the other side of the attacking spectrum, and (in the remakes for Clair) have Pokémon that can self heal off the status.
Meganium has options, but it really has the hardest time dealing with what the in-game teams throw at you. Its weaknesses are constantly exploited, it doesn't have the best speed to deal with threats, and its movepool is lackluster aside from support options. While it is workable, it honestly does have the most difficult time in its generation of debut as Johto hates Grass types with a passion and its options are much more limited when compared to other starters.
A lot of Pokémon forums like Smogon, Pokecommunity and the like actually make in-game tier lists rating how effective certain Pokémon are in an given playthrough. I found that out when trying to look for advice on my second SoulSilver playthrough.