Interesting setups for CO-OP skirmish battles & campaign

Recently got my brother-in-law into Warhammer 1 and he loves it despite never playing any RTS games before. We've been doing skirmish battles, and a few co-op campaigns but I feel like we haven't hit found a good setup to play off each others strengths.

He really likes WE, and I like the Empire. How should we make an army to make use of each factions strengths? I can't seem to grasp how to use the WE, when not using a pure WE army.

One issue we've had during skirmish battles is that during a 2v2 fight against AI, they merge their armies into a supercluster and conduct 1 huge charge against 1 person. It feels like we lose any ability to conduct maneuvers or hold the line. We barely eek out victory against normal difficulty AIs.

One thing I thought would be interesting is the story battles in coop, as those involve waves of enemies and multiple occurrences of reinforcements... but I don't think that's possible for coop.

So tips on fighting together with WE? Might I pick a better race to match him? Ways to handle the AI supercluster?