So I just beat Lagon! It was an awesome fight, especially considering my main skill didn’t work 
I was using a controller and tried to cast smite on Lagon. It would not work. I know smite says it needs a target, so I assume that for some reason Lagon is not a target during his phases? For the rest of the campaign smite auto targeted an enemy I was facing so I assumed it would do the same here. I tried moving the cursor over Lagon and just in front of Lagon as well. I didn’t think to plug in a mouse and try that, sorry. But when I got teleported to fight the tentacles, I could use smite just fine. Once back near Lagon I could not cast it.
I hope this is not intended behavior. Perhaps as a challenge in the Monolith you could intentionally disable a skill, but as part of the story it was an unexpected twist.