I gave up completing bestiary

[spoilerspree]

Less than 1% rare spawn rate are ridiculous. It took me few hours grinding on Boulder area just to find ICU, not gonna waste more time again on this BS spawn rate.

I've grinded metal zone for few hundred levels already with 0 exp jewel, not using exp formation and no exp orb. Met dozens Metal Behemoth, 0 Metal Lobster and 0 Metal Serpent. Tried both metal area (above Izar Port and above Orcus Village) and still no luck.

[Cainohpue]

If I remember correctly, certain Metal monsters only/mostly appear on certain Metal islands. For example, just like you, I fought dozens of Metal Behemoths on that one island without encountering a Metal Lobster or a Metal Serpent, but I eventually encountered them in the other Metal regions (which probably means that there are at least three Metal regions on the world map).