If the BV matters, use the "BV Matching" tab of the "Create Random Army" screen. It is the quickest way to set up a balanced game. Not my favorite, though.
If the theme and background matters, I like to use the "Formation Builder" or the "RAT Generator". I first generate my Lance according to the regiment I want to play with (often Sword of Light but sometimes something very different like... Genyosha or Dieron Regulars :p). Then I generate the opposing team according to whom I want to fight against (and sometimes they are not even Fedrats !), rerolling until the BVs of the two teams are not too unbalanced (usually around 10%-15% of difference is ok).
The RAT tab seems trickier to use, or at least need more patience, but you can set up a team very accurately, especially if you have a specific conflict in mind.