The Gym Leader is a Grass-type trainer. Bring in fire, ice or flying pokemon with the corresponding elemental moves that will allow them to exploit any grass-type's weaknesses. Climb the road ahead to reach the second level. The path to the left leads to a dead end so swing to the right where a trainer is waiting.
Pokemon Ranger Chaise (Prize Money: 2,560)
Climb up to the next level and twice across the nets twice. Climb up to the next level. The rope in the middle going up should be ignored since it has nothing. Swing to the right to find an optional trainer you can fight or avoid.
Pokemon Ranger Maurice (Prize Money: 2,320)
Go back to the previous ledge and go to the left this time. Slide down the rope to the lower level and continue heading west. You'll find another trainer waiting for you.
Pokemon Ranger Brooke (Prize Money: 2,400)
Climb the rope and go up by two levels. You'll find two paths again. Swing to the right and you'll land on the ledge with another Pokemon Ranger is waiting.
Pokemon Ranger Twiggy (Prize Money: 2,480)
Climb up once again and continue going to the right until you find the gym leader, Ramos.
Gym Leader Ramos (Prize Money: 5,440)
Jumpluff deals considerable damage with its Acrobatics move but you should be able to take it out in one hit by using a powerful fire attack. Gogoat is Ramos' main pokemon, capable of dealing huge amounts of damage with Bulldoze or Grass Knot. This damage isn't that high in case your pokemon is strong against those attacks or better yet, they're higher-leveled than any of the gym leader's Pokemon. Weepinbell isn't that much of a threat so just take it out with ease.
Rewards:
The Plant Badge allows Pokemon up to LV60 to obey you and you can now use Fly outside battle. You can now conveniently revisit the previous locations you've been to quickly and easily. You can now access the day-care center, berry farm and Battle Chateau instantly! After the battle, use the slide to exit the gym.
Head to the south and you'll receive a Holo Clip from Lysandre. Continue to the transition gate and you'll meet Sina and Dexio again. This time, they'll upgrade your Pokedex to include Mountain Kalos in it. Head south and exit to Route 13.
Tip: Bring a pokemon that knows how to Rock Smash.