Did you know that you can stack Slow? And not only that, most of the bosses in this game are susceptible to it.
I think this might be the only game in the series where you can stack Slow on an enemy. With your enemy slowed twice and your party on Haste, you can possible take out a boss before they even have a turn.
Also, Zeromus has a very predictable attack pattern that cycles. If you memorize the cycle, you can predict Zeromus' next move. It changes as he loses health, but it's not hard to know when has changed to the next phase.
What are some secrets or tips that you've learned as you've played Final Fantasy IV?