How insanely powerful do you think the Quads, Quints, and Celestials are? Since BeMeebEth is the Quint representing all 5 Ethereal elements, he can harness the full power of those 5 elements, right?
Oooh that's tricky. I do think they're powerful, ESPECIALLY the Celestials and the Colossals, they're basically the Gods of the Monster World, no doubt about that. After them comes Stair Shaper and the Titans.
Now, when it comes to the mons, it's complicated. I think most of the Quints are stronger than the average Non-Quint monster, but they're specifically good with their respective elements, (ex: Candelavra would be stronger using Fire related powers than trying to do stuff with Water), and the 1 elemet they're missing form the 6 should be the one they're the weakest against, (tho it's not like 1 Mammot could take Candelavra down just because she's missing the Cold Element).
The only exception I have to these is Mimic, because my personal headcanon is that Mimic can temporarily gain the Fire Element by eating Relics AND has spent so much time near Fire Monsters that he just adapted that much. But over all, the Quints with the base 6 elements are stronger than the average monster and they're around the same level when compared to eachother.
The Natural&Fire Quads are also stronger than the mons with less Elements, but they are still significantly weaker compared to the Quints.
Now, when it comes to Ethereals, having more elemets still equals stronger abilities, and the Quads are better at using their specific Element, and Bemeebeth can use all 5(although he might be a bit inexperienced compared to the Quads who have been there for longer), BUT! more elements = higher instability! The more unstavle an Ethereal is, the more they need to pay attention to their state, in order to avoid getting hurt!
I also like to think the Ethereal Quints and Quad are stronger than the Naturals&Fires, but that too is due to their instability, meaning they could slip up and use more power than intended, (staying in the Pocket Dimension is both for the safety of others and themselves).
And when it comes to Enchantling, they are stronger than the Magical Quints that have Natural elements.
Basically: Celestials and Colossals are by far the strongest, followed by Stair Shaper and the Titans.
The Quints are FAR stronger than the Quads, and are around the same level when compared to eachother, except Bemeebeth who is poweful, but has a bit of trouble balancing their strenght at times.
Quads are strong, with Enchantling and the Ethereal Quads(who also have a bit of a rare balancing issue) being the strongest ones with their own respective elements, followed by the Naturals, Fire and other Magical Quads.
The more Elements a monster has and the more attuned they are to it, the stronger they are.
