What do i do in harvest moon rune factory after i beat the grimore?

Answer 1

I'm assuming you mean the dragon at the end of the 4th cave, Mt. Gavett (sic?).

The next dungeon is Misty something, and is in the cave surrounded by water one screen south of the intersection. You only need to get the pass from the mayor to get in...Unfortunately, the water only gets out of the way in winter, when it freezes over!

If you aren't in winter yet, then you have some time on your hands. Spend it making money, getting your house upgraded, and romancing your future spouse.

Once your house is upgraded, I'd work on building up your equipment. Particularly your sleeping bag and weapon.

Remember the dungeon is only open in winter, and several crafting ingredients are ONLY available in there, so stock up! (Particularly in fish skeletons, aqua crystals, and the cave's jem.)