For my second game I did the Imperial Guard vs. Orks on very easy, as planned, just to see their tech tree.
Now for the third game I have started the real thing: All four original factions, medium map, medium difficulty. Necrons (me) vs. Space Marines, Imperial Guard and Orks. I'll see how it goes, but this one will be spread over a few days I imagine, instead of taking a single night. I've saved at about turn 22 for today. I'm really focusing on ore mining, since that is the staple food of the Necron.
Guardsmen are pretty weak. Ratlings and Chimeras (Chimerae?) help a lot with the early game. IG are mostly a vehicle faction once you get past the early game.
I have not played Necrons but I think their armor and regeneration makes them relatively easy compared to IG.
They also have the hardest and probably most bullshit questline.
After you do the third (iirc) mission you're pretty much stuck fighting waves of enemies two missions in a row, and the last mission wants you to kill something like 20 kastellans and a techpriest within 20 turns.
Necrons, Orks and Space Marines at least have the decency to have you choose to activate a relic before starting the final mission. You don't get that with IG. It's very much possible to lose the game with them by accident if you do their questline.
I did this mission last week. It was pretty surprising to face so many Castellans, so I'm glad you are saying the Imperial Guard mission is one of the harder ones. Thankfully, I pretty much owned the entire map by this time, and it was on an easy difficulty, so it wasn't too bad. A bunch of Baneblades and Leman Russ converged on them from all directions. I'm glad this one is out of the way, and good to know it isn't representative.
How is the Necron mission?