first u need to have surf. take the ferry to the 7th island
in the trainer tower . it is a reward.
You can get Mantine in the Tanoby Ruins and the Trainer Tower by surfing.
There is no Trainer Tower in Pokemon Pearl. It is only present in Pokemon Leafgreen and Firered and Pokemon Emerald. There is however, a Battle Tower in Pokemon Pearl which is basically the same as the battle towers in Pokemon Ruby, Sapphire, and Emerald.
The strongest trainer in the game besides the one's in trainer tower is your rival is the current champion of the Pokemon league if you defeat the elite four you have the opportunity to defeat him.
If you finished the game and caught every Pokemon in leafgreen there are two things you can do try the trainer tower on seven island or get the diploma and collect all the stickers for your trainer card.
King's rock, Dragon Scale, Up Grade, Metal coat.
You must trade with everygame,go in the trainer tower,and finally catch every legendary.
Kings rock by the way you can earn it from the trainer tower and can be found on one of the islands.
You could battle trainer in there, however, your Pokemon will not gain exp. points. There are 4 mode, single, double, knock out and mixed battle.
There isn't a trainer that has scyther.
Well if your Pokemon are strong enough go north to take on the trainer tower or figure out the mystery of the tanoby chambers in the south.
cheating