No big grass pokemons avaible

If you are really on top with your dodges, you can actually manage to kill a vaporeon with a solar beam parasect. But that's no easy task, that's for sure... Not worth the investment.

Keep farming for the eggs and hope for a few exeggutes with enough candies, it's not that hard...

there's no point use water against water, that only wastes your time

Parasect is good if you dodge, Jolteon too if you have one.

Gyarados if learned no water move.

Exeggutor is one of the best counter but hard to get in many areas

Jolteon is another option.