When I LOSed it, I ported out soon as I saw he was casting it. As far as watching his castbars, I just set him to my focus. I could see everything. I think if you wait until it's just about cast, it's just a case of the spell actual effect and the spell visual effect not lining up -- it looks like you should be LOS, but the spell actual effect has already targeted you. If you LOS sooner it should be a matter of it can't hit you. Others have suggested using Soulshatter right before felpup phase to get his next few casts to target the Pitlord instead of you. I had mixed results with this -- but sometimes, yes, I did get him to cast on the Pitlord instead of me. After that I would try to Soulshatter on cd. You should be able to get two or three total in. Will really help towards the end when you're juggling a lot of stuff.
Generally, however, I would soak the Chaos Bolts. Ember Tap / Pitlord Heal / Healthstone / whatever I had to do to get to full health, then Sac Pact + Twilight Ward. Or Unending Resolve + Twilight Ward.
And I did notice that just because it says he's casting a CB, he doesn't always actually cast it, so you don't have to prep for it unless it's actually casting. Plus, doing the fight enough you know exactly when he's going to cast Cataclysm and when he won't -- using Charge as often as possible while still having it up for Cata + felpups is what got me my kill. Time it to interrupt Chaos Bolts if you need to if you somehow get one up with Sac Pact / Unending Resolve on cd.