Ok. First, you need a super rod. (If you dont have it, go talk to a fisherman in the Fight Area) Next, go to the Resort Area. Go to the lake on the left side. Fish anywhere in the lake and you can get a strong Magikarp (all of them are at least Level 30) Just be patient and you will get a really strong one eventually. There is even a chance to get a Level 100 Magikarp! Plus, if you want a Gyarados, this is a easy way to get one.

Happy fishing.