Hellpoint

This is the true final boss of the game and requires you to complete the Architect's questline in order to be able to fight it. WhiterValkyrie has a good guide for the Architect questline here;

Follow that guide to complete his questline, and after all that is done you need to refuse to give the data to the Interface, defeat it, and then a breach will appear that teleports you to the boss fight with the Sentient. It should also be noted that the achievement Shattered Mirror requires this as well, but with some extra steps so I would recommend doing both the Architect and Prodigal Spawn questlines to knock them both out. DmCTHREE6 has a good guide for the Prodigal Spawn questline here;

As for the fight, for this achievement alone I can't say as I did the one that involves Shattered Mirror as well which means you fight both the Sentient and Prodigal Spawn at the same time. I imagine Sentient does the same exact moves though. I'm going to assume you are doing the double fight, but regardless the same strategy probably still applies if it's just Sentient. Simply just get in close and start beating on him, maintaining some stamina to dodge out of an attack if needed which is usually indicated by the ground turning purple. He also will shoot projectiles from these various things that hover above him so you will need to keep an eye on that. He likes to teleport away so you will have to run up to him again. Assuming you have Prodigal Spawn in this fight too, as you are beating up the Sentient you will want to circle around him to try to keep the Spawn away from you so he doesn't kill you. They both share the same life bar, but the Spawn uses about 1/4 of it and Sentient the rest. So even though the Spawn would die off faster I actually found it easier to focus on Sentient and just try to keep Spawn away from me, as when I attempted to fight the Spawn, Sentient would end up hitting me with some of his ranged attacks as I couldn't really pay full attention to him and they would hurt a lot, some even one shotting me.