Battle Royale III: Frenemy mine.

[Prime32]

Siedfried ends up encasing the entire world in crystal and Astaroth becomes a giant evil thing that brings about the dark ages. (Just checked the ending videos on youtube and learned that I was misrembering what exactly happened)

The "giant evil thing" is still smaller than MegaGargomon, who gets overpowered (and outsized) by the D-Reaper's medium-power spawn.

Freezing the world seems more effective, but it's slow and the D-Reaper is "constantly evolving" so it would come up with a counter eventually. Particularly since the inside of its body follows different physical laws. And its final form generates so much heat it was said the polar ice caps would melt in days.