Going up against Arcanine in Legends Arceus has been one of the toughest battles in the game up to this point. To defeat Lord of the Isles, Arcanine, is a fire type that will rush you and then throw fire bombs at you from a distance. It is important that you keep moving and avoid staying in the middle. Arcanine will do a slam toward the middle every so often and signals it before he jumps. You want to be far away from the center before he jumps so keep running from one side to another. From the rushes Arcanine does, you want to wait just until he has a lock on you, then move to one side out of his path.
Attacking Strategy on the Lord of the Isles, Arcanine in Legends: Arceus
How to beat Lord of the Isles, Arcanine is set with a pattern. The pattern is dodge, dodge, dodge and then attack with the powder bags. After Arcanine is dazed, throw a strong water type pokemon and hit him with a strong attack. So long as your Pokemon is close to the max level you can manage at this point, it should set the next attack phase. After the second round of attacks a fire ring will appear in the middle. You need to avoid going to the middle to not take damage. This will be the final bout, after a string of rushes, Arcanine will jump to the other side of the ring depending on where you are. Will will have to pass to the fire ring to get close enough to hit him so you can hit him with the powder and your final pokemon attack. If you find he has some health left, just keep throwing the powder bags and you should finish him off. Good luck and hope this helps in the battle with Arcanine in Legends: Arceus.