So, This gym will be pretty hard if you chose Charmander as your starter but if you chose Squirtle or Bulbasaur, it will be much easier. Both of Brock's pokemon take 400% damage from Water and Grass type moves which is double the power of a super effective move, so if you have chosen Bulbasaur, I would try and level it up to Level 18 and get it to learn seed bomb. Which should be a good choice against both pokemon. And if you chose Squirtle, try and level it up to Level 15 and let it learn water pulse so it can hit there on its much lower Sp. Defense stat. If you chose Charmander, well, bad luck, I don't believe there is any way to get super effective moves if you chose a Charmander.