For me, the new Alexander DLC and its scenarios are way too difficult… even on Easy. Sure, that’s my problem, but maybe it was planned a bit too big. The DLC isn’t fun if it’s over right away. Just my opinion 
That was already in the Greek-DLC the same (I got only 4 Sessions)
I do think the “easy” difficulty on campaign doesn’t really mean easy but more or less, easier than moderate or hard. As a result, I think there should be a mode that is actually easy. At the same time, being someone who can beat any campaign on hard, I do think it comes down to whether your using a good strategy or not.
There are two new difficulties, one easier and one harder then the normal 5.
Maybe you should try the new even easier one. You can always replay scenarios in higher difficulties.
I don’t remember how far I got–it was the one where you have break the siege. I suck at castle defense scenarios. I stopped there and haven’t been back.
The enemy mostly spams Hoplites with a few Archers so you can just build Lancers and Cavalry Archers. Macedonians have all Castle Age Archer upgrades including Thumb Ring.
That mission is certainly a difficulty peak though.