A level 7 gym has very high prestige and there are 7 defenders. It is not easy to take such a gym down. In the video below, the player defeat the gym with his attackers.
The 1810 CP Gyarados uses Dragon Breath, a dragon-type fast move. It is not super effective against Golduck, a water-type Pokemon, but the move damages much due to the Gyarados's higher CP.
Since the HP of Gyarados is not full, it is not easy to win. Additionally, the Golduck uses Hydro Pump, a water-type charge move. However, the Gyarados wins this battle.
Even though the Gyarados uses Hydro Pump, its charge move, it gets injured already and the defender, Snorlax, defeats the first attacker.
Slowbro uses Water Gun, and the fast move is famous for its high DPS. Althoough the Snorlax uses Earthquake, the second defender falls in battle.
The fast move of the Golduck is Confusion, but it is no effective against Slowbrow. The Slowbro beats the Golduck easily by shooting Water Gun continuously.
The fourth defender is a Pinsir. The fast move of it is Rock Smash. However, the CP of the Slowbro is higher than that of the Pinsir, and the attacker wins this battle.
The Slowbro's Water Gun is not effective against the Lapras because the defender is water/ice type Pokemon. The fifth defender uses Dragon Pulse and beats the Slowbro. The next attacker, Golduck, finishes the Lapras off.
After beating the Lapras, the third attacker, Golduck, fights against the Gyarados, the sixth defender. The Gyarados uses Dragon Pulse but is beaten by the Golduck.
The last defender, Arcanine, beats the Golduck down by using Bulldoze, the ground-type charge move.
After that, the Golduck, the fourth attacker and the second Golduck, goes to the battle. The Golduck has higher CP than the last one and defeats the Arcanine by using Water Gun, which is super effective against the defender.
