Ramuh.Mizuharu said:
To be honest, I wouldn't mind another wyrm or some dragon-type monster(s)
Ruby Dragon.
What made this monster deadly was not so much its high level, HP, or powerful attacks/magic; but rather its intelligence and ability to self-preserve and counter whatever you throw at it.
Not many mobs were smart enough to cast reflect on themselves, and subsequently nuke themselves to have it bounce on you making it impossible to magically attack them without doing the same thing and wasting precious resources and time.