There are actually 4 moves you learn from Sentinal that will allow you to raise your shield, blocking damage and allowing you to use Phalanx/Phalanx II.

Level 1 - Guard :- Readies your shield. During this time, you have a chance to partially block (reduces damage) or block (eliminates or nearly eliminates damage).
Level 10/Level 40 - Aegis Boon/Aegis Boon II :- Recover behind the safety of your shield, increasing defence against frontal attacks and converting blocked damage into HP
Level 20 - Outmanouver :- Strategize for survival, increasing defence against frontal attacks and increasing TP generated by blocks.
Level 50 - Deflection :- Focus on your shield arm, increasing defense against frontal attacks and negating a portion of blocked damage

As I said before, each of these allows you to use Phalanx/Phalanx II after blocking damage. The only one thats hard to spot is Aegis Boon/Aegis Boon II, as you may have noticed you don't get hit, you get healed, when you see The "insert Name Here's" hit heals you for XXX Hp, you can then use Phalanx/Phalanx II.

As a side note, Shield don't block TP moves from mobs, well at least as far as i can see they don't, and Phalanx/Phalanx II won't work after TP moves either.