Zeromus' full stats can be found here.

A description of all of Zeromus' spells can be found at the end of this article.

This repeats until the Crystal is used.

After the Crystal is used, Zeromus' sprite changes and he starts his initial phase.

NOTE: The power of this first Big Bang is not explicitly set, but Zeromus' magic power defaults to 32 (9 multipliers).

Because the Virus phase technically starts as soon as Zeromus queues the first Big Bang, the counter table switches to the next phase's at that point. This includes the HP check when direct damage is inflicted.

If, at any point during this phase, Zeromus takes direct damage (or MP drain), and he's taken 45000 damage or more, he refills his HP to full (61000) and enters the Nuke phase. Otherwise, this phase repeats.

If, at any point during this phase, Zeromus takes direct damage (or mp drain), and his health drops to 12000 or lower, he enters the Meteo phase. Otherwise, this phase repeats.

Repeats until the fight ends.

Normally it takes one turn for an enemy to queue a command and one turn to execute it. This is what allows Big Bangs to be nerfed: Zeromus' magic power is set when the spell is queued, but can be modified by provoking a counter-Nuke before the spell is actually executed.

However, if an enemy script starts a chain, all other battle participants' timers are immediately frozen and all actions in the chain are executed in turn. This allows an enemy to act more quickly and without any possible interruption.

This also means Meteo's spell power cannot be modified (buffed/nerfed).

Nerfing works by taking advantage of counter-attacks to modify an enemy's magic power stat before it casts a spell. Since Zeromus sets his magic power to 31 or 32 (8 or 9 multipliers) when casting Big Bang, but sets his magic power to 14 (4 multipliers) when firing a counter-Nuke, a well timed counter can lead to a Big Bang with only 4 multipliers. This drops the maximum possible damage from 2400 or 2700 down to 1200.

Because Zeromus does not set his magic power before the first Big Bang, it can be nerfed at any time. All that matters is that the attack leads to a counter-Nuke. (Crystal will always work; for anything else you have to wait for Z to use his first turn to become vulnerable)

Subsequent Big Bangs can only be nerfed after Zeromus has queued the spell, which makes the timing extremely tight. Due to the number of factors that go into this timing, it is beyond the scope of this article.

Any direct-cast (not reflected) spell that can hit Zeromus will trigger a counter. This includes most j-items and weapons used as items, as well as the Crystal.

NOTE: Keep in mind that Holy magic (White/Meteo) cannot nerf a Big Bang because it is countered with Weak instead of Nuke, and in most phases¹⁾, Rydia's summons can actually buff a Big Bang by invoking a 10 multiplier Virus counter.

Any spell that checks the boss bit will not affect Zeromus in any way. This includes the items HrGlass, MuteBell, Coffin, Bestiary, DietFood and Life Potions.

Heal potions and the Heal spell will trigger a counter-Nuke, but will also reset Zeromus' speed modifier to the default, negating any prior Slow or SilkWeb usage.

Anything that deals direct damage to Zeromus' HP or MP²⁾ will trigger an HP check. If Zeromus is at or below the HP threshold for his current phase (16000 HP in Virus phase, or 12000 HP in Nuke phase), this will lead to a phase change.

NOTE: HP checks take priority over counters. This means that if a spell causes Zeromus to tip into the Nuke or Meteo phase, there will be no counter-Nuke, and thus no nerf if a Big Bang is already queued. If a phase tip is imminent, using non-damaging spells/items such as the Crystal can ensure that a Big Bang is nerfed.

Zeromus only refills his HP and moves into the Nuke phase if he takes (non-lethal) direct damage³⁾ and his HP is at 16000 or less. Reflected spells bypass this check, allowing for much quicker kills. Reflect strats can be safer too, as Zeromus will not enter the Meteo phase.

Since the HP refill check does not activate until the first Big Bang is queued, a party with extremely high damage output can actually kill Zeromus without relying on reflected spells. However, any non-fatal direct damage after the Big Bang is queued will cause the HP refill (and transition to Nuke phase) to take place.

One way to know when Zeromus changes phases is by counting damage, but there are also visible signs of the transition.

When direct damage tips Zeromus into the Nuke or Meteo phases, it does so by triggering a counter script that is similar to a normal “Do nothing” turn. This script can take a few seconds to run, leading to a noticeable lull in the fight that is indicative of a phase change.

If a direct damage spell does not incur a counter, that also means the spell itself pushed Zeromus into the next phase.

A “fake shake”, where Zeromus shakes but never casts Big Bang, means that Zeromus changed phases in between the shake and the Big Bang. Two shakes without a Big Bang in between are a sign that Zeromus has entered the Nuke phase.

Target: Entire party
Effect: Removes Wall, Barrier, Image, Float and Berserk status, as well as resetting the target's speed modifier to its default value.⁴⁾

Effect: Very scary. Does nothing.

Target: Entire party
Multipliers: 8 or 9 (modifiable by inducing a counter)
Effect: 200-300 non-elemental damage per multiplier, and Sap (HP leak) usually lasting 30 ticks.

(Identical to Disrupt)
Target: Entire party
Effect: Removes Wall, Barrier, Image, Float and Berserk status, as well as resetting the target's speed modifier to its default value.⁵⁾

Target: Single party member
Multipliers: 4 (Scripted cast: modifiable. Counter: not modifiable)
Effect: 400-600 non-elemental damage per multiplier.

Target: Entire party (split damage)
Multipliers: 10 (Scripted cast: modifiable. Counter: not modifiable)
Effect: 128-192 non-elemental damage per multiplier, divided by the number of targets.⁶⁾

Target: Entire party
Multipliers: 1 (not modifiable)
Effect: 800-1200 non-elemental damage per multiplier.