Well I inadvertently angered him by dropping a nuke on a Machine outpost that I couldn't keep clear and I was struggling to hold on to when I did. It was the centre-piece of my fight with the Machines. After getting crushed by the thing in my first attempt at this Cults game (I refused to give him the humans he wanted) I pacified him this time and made sure I was producing a lot of human resources.
So fast forward to me nuking this outpost and the two or three armies in it (killed a few of their heroes too) and all of a sudden I get the message saying he's been angered. He thinks I was aiming for him, apparently.... uh-oh.
I hadn't got enough other resources to build another nuke munition (and I didn't know whether that'd work anyway) and I was struggling to get the best units. So I used Ulysses Starke and cobbled together an army that would have held it's own against normal units but I wasn't confident against this thing. I created a tactic nuke attachment and gave it to some infantry with decent defense and then waited for him to come to me.
When he attacked I just hoped I got the initiative, which I did, and used the tactical nuke on him hoping for the best.
It worked, I killed him in just one turn