A detailed description of Zeromus' script and abilities is available here.

Zeromus
HP XP Gil Boss Level Attack Element
61000 0 0 true 96 None
Attack Accuracy Defense M.Def Speed Spell Power
174×13 99% 1 17 69 to 69 32
Resistances
None
Weaknesses
None
Race
None
AI
Condition 1 If condition flag is 5
Start a chain
Set spell power to 3
Do nothing
Chain into
Target all characters
Cast Meteo on single
End chain
Wait
Do nothing
Condition 2 If condition flag is 4
Do nothing
Wait
Use [screen Shake Effect]
Wait
Set spell power to 31
Use Big Bang
Wait
Use Blk.Hole
Show message 118
Wait
Set spell power to 14
Cast Nuke on single
Wait
Do nothing
Wait
Do nothing
Wait
Use [screen Shake Effect]
Wait
Set spell power to 32
Use Big Bang
Condition 3 If condition flag is 3
Do nothing
Wait
Use [screen Shake Effect]
Wait
Set spell power to 31
Use Big Bang
Wait
Use Blk.Hole
Show message 118
Wait
Do nothing
Wait
Use [screen Shake Effect]
Wait
Set spell power to 32
Use Big Bang
Wait
Set spell power to 36
Target all characters
Cast Virus on single
Wait
Use Blk.Hole
Show message 118
Wait
Do nothing
Wait
Use [screen Shake Effect]
Wait
Set spell power to 32
Use Big Bang
Wait
Use Blk.Hole
Show message 118
Condition 4 If no condition
Use Dummy
Wait
Use [screen Shake Effect]
Wait
Use Big Bang
Set condition flag to 3
Reaction
Condition 1 If condition flag is 4 and takes damage and self has 12000 HP or less
Set condition flag to 5
Do nothing
Condition 2 If condition flag is 3 and takes damage and self has 16000 HP or less
Set condition flag to 4
Use Vanish [Restores HP to Full]
Condition 3 If a character uses Call and condition flag is 3
Set spell power to 14
Cast Nuke on single
Condition 4 If a character uses Call
Set spell power to 36
Target all characters
Cast Virus on single
Condition 5 If a character uses Holy magic
Use Weak
Condition 6 If a character uses magic
Set spell power to 14
Cast Nuke on single