It was on Rookie, given it was my first go at things. I dragged this out for longer than I needed because I was having a lot of fun, but at a certain point I ran out of things to research and was just waiting for missions to appear lol. The Avatar timer pretty much gave up at that point too, so there wasn't actually anything pushing me to the end.
Talking of that final mission - what a massive step up in difficulty. Not because the units are different, but because there were so many of them! I realised too late that the mobs were just going to keep spawning in the final part of the mission too, and that I should have focused all my efforts on the Avatars; I ended up with two Avatars on the field at the same time by the end. I'm amazed nobody died!
Now a question for the veterans of the game - should I start a new campaign at the next difficulty up, or start a WotC campaign? I've heard WotC is quite a step change.