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Exp. share is great and all but I dislike pokemon that heavily rely on it. This is that worst case scenario where even going through the Elite Four would still take a long time.
The other pokemon mentioned can also still learn attacking moves, cosmog can't. Any pokemon with a damage-dealing move, even those with lower base stats like sunkern, would be better off in a fight.
It's pokedex entries make it sound like something awesome came- a solid gold-shelled pokemon fueled by starlight that ancient rulers built altars to, and called cacoon of the stars. It even got this huge defense stat buff + sturdy...but then you look and see the only new thing it will learn is cosmic power, and your heart sinks...
Concealed within his fortress, the lord of Mordor sees all. His gaze pierces cloud, shadow, earth, and flesh. You know of what I speak, Gandalf: a great Eye, lidless, wreathed in flame.
leveling up Cosmoem is even harder because you obtain it at level 43. It needs 10 more levels. This is worse than trying to level up something like Dragonair to Dragonite.