As both opponents are on different sides of my empire, I have found it fairly easy to select forces suitable for fighting the two very different enemies. Against the Skaven, I have to face lots of big units of infantry, with frenzied plague monks, infiltrators and warmachines. I use lots of template weapons like the flamecannon and stone throwers to cut the skaven rank bonuses down. I also use small units to protect my artillery from the infiltrating gutter runners. These can pick off unguarded crews, but don't do to well against a proper fighting force.
Chaos armies tend to have a few expensive troops, with large very expensive daemons.
Against these I deploy my heavier guns, the cannons and organ guns. I also give all my characters weapons that do multiple wounds, as most chaos troops have at least two wounds, and lots of Big Kill items like Frostblade, Rune of Death and so on. Another bonus for designing campaign armies is the ability to specialise banners. Although we limit each banner size to 2000pts, banners can be used to support each other and several banners can join in battle to overpower the foe. These are my main, large banners, up to 2000 pts, Presently I have a Longbeard banner, Hammerer banner and an Engineer banner.