i would try an electric type, that mostly works but sometimes random pokemon work so try to find a pokemon like lanturn or raiku.
You get Hm flash from the hiker in granite cave in dewford.BEWARE!You can only use flash if you get the knuckle badge from Brawly
Use the hm surf
You have to use a HM called strengh.
there are many MANY Pokemon that can use the hm flash in Pokemon sapphire. the following Pokemon can: grovile( treeko),kirlia(ralts)shroomish, eletrike, Roselia, plusle, munun, magnemite, voltorb and many more
HM 05 is flash
In Pokemon Platinum, Flash is a TM.
Look at "How do you get hm flash in blue?" On this site.
on Pokemon yellow on Pokemon yellow
Flash is in the Granite cave. Talk to the hiker guy as soon as you walk in the door. He should give it to you.
Teach a move to a Pokemon that can learn it via HM and then, after defeating Falkner in Violet City, select that Pokemon in your party and select Flash to light up dark caves, such as Dark Tunnel.
its not an HM anymore you have to find it.
Find the HM (in the building next to the Pokemon centre in rustboro city) and use it in the menu