Even though you will face Kahuna Hala early in the game, thus making his Pokemon not all that strong, he can still be one heck of Kahuna, an opponents to defeat this early. If you are having difficulties with him or want to avoid such a thing before facing him, you should first consider raising all of your Pokemon to at least Level 20. Kahuna Hala's highest leveled Pokemon is Crabrawler at Level 16, which would give you the edge before the battle even begins.
Another thing you can do to have an advantage over Hau's Grandfather is trade for the Hawlucha in the Route 2 Pokemon Center. Not only does Hawlucha bounce off the Fighting Type Attacks Hala's Pokemon like to deal, but its own Flying Type Attacks are Super Effective against each of his Pokemon. Hawlucha is also really fast, helping you outspeed all of his Pokemon while in battle. With higher leveled Pokemon and the help of Hawlucha, Kahuna Hala is a cinch to defeat.