Final Fantasy IV Playthrough - EP22: Battle for the Blue Planet
4Two Phase enemies in one fight!? Yeah, no.
On the bottomost floor you can only fight two special enemies:
ZemusBreath - Only uses Libra to scan you which is annoying but that is it.
ZemusMind - I did not fight this one but it only spams Shell and Protect on itself.
The two are harmless enemies you can fight for a lot of EXP if you need levels before the final boss.
As for Zeromus, his AI is actually quite complex for an old game but some of it is something you cannot know. He also sports impressively over 65000 HP.
Big Bang is his signature attack and his way of wiping you out very easily. If you cannot survive Big Bang at all then you are not ready. If you strike him with a normal attack when he shakes, his next Big Bang will be weaker so you do have a way here but you need to know this. It has to be a shake followed by Big Bang though.
For most of the fight, using Rydia is tricky because he loves to counter summons and Black Magic with powerful magic of his own until his last phase so I played it safe.
When Zeromus starts spamming weaker Meteo, he is close to death.
Black Hole nullifies all buffs and in reality Protect and Shell do not work at all in the fight but you do not see this. I read this in reverse engineering notes by ROM hackers in the past but if it is really true I cannot say.
Finally, the biggest eff you. If Zeromus gets a turn between 16k-12k HP he will also recover his HP back to full and switches to slightly more frequent Big Bangs. Again, you do not know this happened. On the other hand, your damage output should be more than enough to get him below 12k by simply playing normally without him getting that turn and thus forcing him to skip this phase.
And that is that!