The Rune Bow can be found a bit past the door where Hein awaits. Hein uses Barrier Shift in lieu of any other action on every third round, and begins the fight with a random weakness. Hein is almost impossible to hit physically, so your best bet is to have your Scholar Study him on the third round (hoping he will Barrier Shift before the Scholar goes), and then again on the next round if you're still unsure of his weak point. Black or Red Mages should cast their most powerful Spells of the appropriate Element, while Scholars should use any elemental attack Items you have during the intervening rounds to make use of their high Intelligence. Physical fighters won't have much to do in this fight (it may even be worth using multiple Scholars for this reason).
Note that Barrier Shift will always make Hein vulnerable to Fire, Ice, or Lightning, and cause him to absorb all other Elements (including Holy and so on), so plan accordingly. There is no guarantee that Barrier Shift will actually change Hein's weak point, either.
Note that Barrier Shift will always make Hein vulnerable to Fire, Ice, or Lightning, and cause him to absorb all other Elements (including Holy and so on), so plan accordingly. There is no guarantee that Barrier Shift will actually change Hein's weak point, either.