Arcanine is one of the everyone's favorite Pokemon due to its high CP. You may have encountered Arcanine many times in gym battles. Thefore, knoing the ways to beat Arcanine is importanto for winning gym battles.
Vaporen, evolving from Eevee, is well known as the best water-type Pokemon due to its higher CP and HP. When Vaporeron can use water-type moves which Arcanine is weak to while reducing damage of Arcanine's fire-type moves.
The combination of Water Gun and Hydro pump is an optimal moveset for Vaporeon to beat Arcanine based on DPS and EPS.
When Arcanine's charge move is Bulldoze, a ground-type move, Gyarados is the best to fight against it. Gyarados can reduce damage of Bulldoze (Bulldoze is not effective against flying-type Pokemon), its Hydro Pump is super effective against Arcanine.