What matters is the type of the moves you use, not the type of your pokémon.
Sneasel is weak to Rock and Bug attacks.
about level 40 but use a grass or electric type Pokemon
Really, any Pokemon can beat a water Pokemon if its strong enough but the easiest way is to use a grass or electric type that has a same or greater level than the water type.
You may need to level up your bulbusaur to level 16 then retry to beat charmander, try catching other Pokemon to fight charmander that are not grass types, advice get a geodude.
Any powerful grass-type Pokemon for example cacturne, Roselia, breloom, shiftry, tropius, sceptile.... Just make sure it's at a reasonable level and has good grass moves.... Have fun!
Fire type has the very strongest advantage over Grass type Pokemon
sneasel learns it at level 57
Sneasel learns Beat Up I think at level 37.
Sneasel. If I remember correctly, Beat Up is Sneasel's signiture move. In FireRed/LeafGreen, you can't catch Sneasel until after you beat the Elite Four.
none of the fifth generation Pokemon learn it but in black and white sneasel learns it at level 42 houndoom learns it at level 26 and houndour learns it at level 25.
about level 40 but use a grass or electric type Pokemon
What you do is train up another pokemon and yours to a high level in the grass
after you beat the elete four go in tall grass and you will find level 50 Pokemon there.
You can beat Skyla the gym leader with a higher level pokemon or with electric or rock type pokemon. And I do not reccomend using grass or bug types.
you need to level up your Pokemon, and it depends on what type of Pokemon they have. for example if they had a grass Pokemon the best thing you should do is catch a fire Pokemon or level up the fire Pokemon you already have.
any grass or dragon moves will be good (the higher the level of pokemon the more damage the moves will do to the target)
get a Pokemon that is strong against water. like: Electrick and grass. then train it so its about level 20 and knows the types tht work well with it. then get Pokemon to beat her/his other Pokemon she has.
Pokémon that can defeat Sneasel are Fire, Fighting, Bug, Rock and Steel types such as Magmar, Machamp, Scizor, Pinsir, Golem and Onix.