Normal difficulty, since I don't have any fun from enemies getting arbitrary buffs instead of better AI. And I don't think they dodge any better on higher difficulties...
Dwarf engineer also provides army buffs to ranged including improved range, dmg, reload time and ammo.
I haven't played skaven much, since all their good units are behind a paywall. They might very well be deadlier in their offensive capabilities (especially with ambush), but dwarves will still hold their own if enemies do get through while skaven just run and/or die.
On chaos:
Yeah, they are currently weirdy weak. But there will be several invasions. On my current playthrough with von Carstein, Archeron came in a second invasion around turn 170. (And was rofl-stomped by Manfred and I). Also weirdly, the chaos LL didn't give traits after beating them.
There are mods that buff the invasions again, but I haven't tried them.
On "winning" with beastmen:
Armies should always work in tandems. One strong army in secret raiding camp. One weak army in the open as bait.
Use the lord with the highest income buffs (sacking, post battle loot, etc) for high value targets. Use the secondary armies for low value target for training.
Focus on armies and non-walled settlements in the beginning.
Mostly ignore non-Empire/Bretonnia factions. If possible, even ally with them. Since Grimgor snowballs really bad with the current update, he'd make a good candidate.
Then it's just a matter of kicking Empire/Bretonnia often enough so that others take over their settlements. (I had a weird campaign, where I was fleeing HE while razing Empire. In the end, HE took over all Empire settlements.)
With alt-click artillery might actually be useful...