Heatran

Before continue doing this "quest", I expect you have already obtained Magma Stone at Route 18.. If not, you might want to read my P2 & Colress post.. If it's a checked, then fly to Yamaji Town.. Enter Reverse Cave from here..

Then go to where Bianca leaded you to last time.. If you can't remember, head downward from here till you see an end with an entrance but don't bother about it.. Now move right.. Shortly, you will see a nurse standing beside a stair.. Go down the stair and you will be asked to use Magma Stone. Use it and Lvl 68 Heatran will appear.. This is gotta be the shortest post ever.. XD