I finally killed the Sentinels in Bastille. It was a lot easier solo than with "helpers." I went in and made a few attempts and got the pattern down pretty good but was running low on Hollow health (never got the ring that stabilizes health at 75%) so I helped out on The Pursuer to become Human and reset my health. Went back and figured "I know the fight now so let's just summon some people in and trash these guys." Well the people I summoned weren't the best... We killed the first on the ledge pretty easily, dropped down and they both, promptly, got killed. So I spent the next 5 minutes or so soloing the remaining two. I killed the second and got greedy on the third and he hammer slammed me. Went back as Hollow and killed them solo with ease.
The worst part about my story is that summoning the helpers hurt me a lot more than anything. If I hadn't summoned them, I would have won. It seems that when you summon people in, your damage output goes WAY down (I was hitting for half of what I normally do). So having to solo two Sentinels with the Multiplayer penalty was too much for me. Once I saw that, I figured it was easier to just do it solo since I knew the fight really well by that point. Forget who it was but the person a couple pages back who posted the strat of killing the Ledge one first followed by a lot of circle strafing, thanks. That's exactly what I did.