This Seeking Tech guide will show you how to beat the last boss in Final Fantasy Pixel Remaster.
You will do more damage against Chaos by using physical attacks instead of magic attacks. You can use the Haste and Temper spells on your main attackers to increase damage.
You can also boost your defense by using the Protera and Invisira spells.
If you are having trouble, then you can try grinding for a few levels before trying again. At higher levels, Chaos shouldn’t be too difficult to deal with.