How do you get strong enough to battle DAD?

You can battle your dad after you've beaten the 4th gym leader, which is Flannery in Lavaridge town.

Well you have to beat dad to get surf but to battle him you need 4 badges hes the 5th gym after you beat him you go to wally,s house and you probably get surf.

need to get the first 4 gym badges, then go to petalburg city, there you will find the gym in which your dad owns, then do as you usually do in gyms, then once you find him in the gym you battle!! But becarful!! He has tricky Pokemon, which, are easy to beat, if you have the right type and if your at the right level! Then once you beat him, go out turn to the left and go to the house, and if you talked to everyone like you are supposted to do, then youd find out that wally is looking for you and wants to give you something, but any way, go in the house and talk to the dad! He pretty much says that he is happy you helped his son and BAMM!!! You have surf!!!! And if you are interested in what wally wants, talk to his mom, and then you find out that he left his unkles house without telling anyone!!!! But she says that he'll be ok.

The best types of Pokemon use to beat Norman, aka DAD, are fighting type or ground type. My personal preference is to use Lairon at Level 30 that has been taught the TM Earthquake. But any Pokemon that knows other ground type attacks are also helpful in wearing down Norman's Pokemon, especially his Slaking.

Hope this helps.