The Ruin Sentinels can be made optional by utilizing the Pharros' Lockstone mechanism accessed early by means of the Antiquated Key. However, skipping them means that the player cannot access Belfry Luna nor various areas locked behind gate mechanisms.

Can I skip the ruin sentinels?

Ruin Sentinels boss skip: This skip requires the Exile Holding Cells bonfire and a Pharros' Lockstone. ... The further of the two secret rooms will open and have an elevator cage to take you up to the entrance to Sinner's Rise, or you can backtrack down the staircases to the Servants' Quarters bonfire.

Is Flexile Sentry mandatory?

Flexile Sentry

This boss is optional if you go to Lost Bastille from the Pursuer area in the Forest of Fallen Giants.