1 HP: 8985
Scarlet, or Amarant, has an HP of 8985. This battle could be a little difficult if you aren't well prepared.
2 Hi-potions needed
You need a stock of Hi-potions when fighting that boss since there will be nobody else to heal you when you get hit. At this point, ordinary potions won't be useful.
3 Equip running shoes
If you have running shoes then equipping them will give you auto haste. In such a case, your turn will come more often and so winning this fight will be much easier.
4 Equip Brigandine
Brigandine is a kind of body Armour that could provide you with good physical defense. Since all of Scarlet Hair's attacks are physical, equipping that Armour can help you a lot.
5 Equip the power belt
Equip the power belt if you still have it to enable the MP attack ability. This ability can help you do a lot of physical damage easily.
6 Avoid hitting him when he is in the background
When he goes in the background don't attack him for he will counter back with a physical attack that is much stronger than his normal attacks. (See How to defeat Ralvurahva in Final Fantasy 9?)
7 Attack him near the third pillar
If you attacked Scarlet Hair while he is behind the third pillar from the left then not only will your attack cause a lot more damage, but he also won't counter.
8 Enable the man-eater ability
The man-eater ability can also help you cause more physical damage when you attack him. Make sure it is enabled.
9 Items to steal
This boss has both Poison Knuckles and an Ether. You can only try to steal the first since it's a bit expensive. (See How To Win Final Fantasy 9 Card Game)