Snake Attack
You met him when trying to cross the Bandit Butte. You have to fight him in the Sunken Tower and in the Star Tower as well.

Dure to the fact that his attacks as Phantom Sentry and Star Sentry are nearly identical, I combined them on this page.

On the left you see the Snake Attack. This attack blocks any counters so use this to heal yourself.

Killing Fields / Slaughter Slice
Gloomy attack, stands alone when you confront him as Phantom Sentry, will be followed by Mile High Attack when Hiro fight him in the Star Tower (Mile High Attack: see below).

Aura Wave
Phantom / Star Sentry normaly uses the Aura Blast as the first strike against you.

Mile High Attack...
...will be used by Star Sentry only.

Maybe you noticed that he changed his outfit slightly.

Nevertheless it won't help him much. No one can stop Hiro on his way to see Lucia.

Tip:
Use Althena's Sword to heal Hiro from time to time!