Continue your journey with Prima's Dark Souls 2 Guide and Walkthrough!
Reward: Flexile Sentry Soul
There are three keys to defeating the Flexile Sentry. The first is to equip a weapon that can be used from a distance. A lance, spells or anything that doesn't require you to be right in the face of the enemy. The second key point is to use the mast in the middle of the room to prevent the Flexile Sentry from getting too close with its jumping ranged attacks.
The third, and possibly most important key, involves taking down the Sentry with moderate speed. As the fight progresses, the water in the room gets higher. The higher the water level, the harder it is to move around. This makes it far more difficult to evade the Sentry's attacks. If the water level gets too high, the fight is essentially impossible to win.
The Sentry has two attack types depending on which half faces you. One side attacks with two spiked clubs, while the other attacks with dual swords. Both sides have similar attacks, but the side with the clubs is the easier of the two, due to the fact that it uses long-range attacks less frequently. If you can maneuver around the room to face the club side as often as possible, it makes this battle a little easier.
Use the mast in the center of the room as a pseudo-shield to stop the Sentry's long-range attacks. In most cases, the Sentry attacks with a single horizontal swiping attack, or two vertical attacks. It may end an attack series with an additional double club attack. This attack covers more range than any of the normal attacks (aside from the long-range jumping attack), and has a considerable amount of recovery. When you see this attack, quickly counter with at least two attacks of your own, or more depending on the type of weapon you're using.
Move around the mast in the middle of the room, but time is a factor in this battle, so allow the Sentry to get close enough to attack. When this happens, move away to avoid the attack, then counter with your own attacks after the Sentry attacks at least twice. It rarely uses a single attack, so always wait for at least two attacks before you begin attacking. Be cautious of the third attack that will occur if you're too close to the Sentry. It inflicts a considerable amount of damage, but as previously mentioned, it also has a great deal of recovery.
Once the Sentry is down, head through the doorway and up the ladder at the end. Open the coffer in the corner of the room to obtain a Pyromancy Flame and a Fireball. Examine the circular object in the far corner to watch a cut-scene. At the conclusion of the short cut-scene, head down the pier and to the left, then through the corridor and into the room ahead.
Take the lift to the floor above, then look in the middle cell on the left to find the Soul of a Proud Knight and a Radiant Lifegem. Head through the doorway and up the stairs to enter the Lost Bastille.