Flexile Sentry Boss Guide – Dark Souls 2 Wiki Fextralife
Most of the Flexile Sentry’s attacks are impossible to dodge unless you have very high adaptability. Thus it is better to block the attacks with a shield, such as the Drangleic shield, that blocks physical damage. You should roll out of the way off his quick slashes and then get back into range to attack. You must finish this fight quickly as the room fills up with water and your movements will be slowed down.
The strategy is to stay close, walk left and wait for a heavy 2hit combo. If you are standing to the Sentry’s left the second attack won’t hit you, giving you a chance to counter. When you are done with the fight, remember to pick up the pyromancy..
As found on Youtube