Difficulty matters, for Normal Campaign and hard I used different COs.

NC:OS: I use Javier and Jess, no tagging just push Javier's direct defence up using skills also for mobility use prairie dog and pathfinder, give Jess Teamwork, Bodyguard and Synergy.

BM: Colin and Sasha, Either use Colin for units that cost almost nothing or Sasha to stop Vonny from using his SCOP, whichever path you take use all the money skills on the main CO and the same skillset as Jess on the other CO.

GE: Eagle with Sami, give Sami the same skillset as Jess, for Eagle do whatever to make him stronger.

HC: The only change to Hard Campaign is the OS COs, Kanbei with all defence skills and Sonja with the same skill set as Jess.

Sacrifice your starting Megatank, break the cannons and rush'em.