I've been playing Rune Factory Frontier, and it's been pretty fun so far. I've only fought one boss, but it was a damn difficult one.
The boss of the first dungeon (Green Ruins) is a giant Gryphon. The thing does 60 damage, easily, can fly, keeping him out of range, and even spawn chicken enemies of its own (fortunately, it also kills them). It has a charging attack that can cause it to become dizzy if it runs into a wall, but late into the battle when it gets its attack/speed boost it becomes much more difficult to get it anywhere near the walls. Oh, and it starts doing around 80 damage per attack.
It's a shock, especially since the DS games were so much easier in terms of boss fights. Bosses can be fought again as well, and I think their power either scales with the player's level, or they just get stronger each time they're killed. I deduced this when, after I defeated it once and fought it again, it was no longer weak to fire attacks. Either way, I wasted a lot of potions, only to die in the home stretch.