My usual starting move as Traitors in BaB scenario is more troops for Angron in the south, otherwise the Imperials will surround him (White Scars joining in) and very likely kill his entire posse. That is unless the bombardment/corruption opened a retreat path. I always target the area with two Titans + two tanks (between the factory and the spaceport), looking to eliminate the Titans at least throught bombardment, if I can get them and corrupt the tanks, win-win.
As Imperials, unless Angron wasn't supported, Boarding Action to VS strat map usually for one point, then another order, possibly the movement one that doesn't place an activation token if executed from the strat map.
Fulgrim comes out next, usually to the south-east Spaceport (hopefully the defense laser in the fort has been removed via corrupting the defenders/coexistance battle). That way, any units still in the Spaceport have to face a corruption check in the coexistance battle. If Angron has been supported on the first action, this gives the Traitors three of the four spaceports already, could even be 4 for 4 if corruption went perfect in the north-east one.
For scenarios 2-6 (haven't tried #6 yet, but IIRC, unit setup is the same as in #2 for all the rest), as Traitor, take the north-west Spaceport first, it's the closest one to the Inner Palace (the one where Mortarion starts in scen #1). This usually leads to the Imperials taking the north-east one, they don't want two Traitors holds right next to the Palace. Place Mortarion + Magnus with their Chaos Space Marines at the north-west spaceport, they make a formidable team, Mortarion dropping combat levels and Magnus bombarding. The southern Spaceports can easily be taken with Angron/Fulgrim, since they usually hold just Imperial Guard/Tanks (you don't want to leave the Titans or Space Marines out in the open to be bombarded as the Imperial player, so they are stuck in fortifications/factories).