It's buried at the bottom of Boss Battle Strategy:
"Since the Dream Devourer has 32000 HP, which is close to the signed 16-bit integer limit, it is actually possible to defeat it by healing it. By using powerful magic that it absorbs during its second phase, it can be (temporarily) healed past 32767 HP, which overflows the counter and makes its HP go negative."
"Since the Dream Devourer has 32000 HP, which is close to the signed 16-bit integer limit, it is actually possible to defeat it by healing it. By using powerful magic that it absorbs during its second phase, it can be (temporarily) healed past 32767 HP, which overflows the counter and makes its HP go negative."