Here's the japanese description of focus fire:
I believe it roughly translates to:Code:集火値とは、一種のマークのこと。 集火値ついている部隊が直接攻撃ダメージを喰らった場合。 集火値が溜まった分、ダメージがそのパーセント増大する。 集火値はターンエンド毎にリセットされます。
Focus fire is a type of mark.
Focus fire is applied through direct damage. (Applied to a single target only, I guess. Could also be saying aoe attacks don't benefit from it?)
Focus fire's accumulated amount increases damage by that percentage.
Focus fire is reset every round.