Use Bear on every combat-heavy mission. Use his skills to upgrade his armor first and foremost.

Archer is necessary to keep your people cool under fire. Upgrade his leadership and accuracy skills and use him on point.

Select the sniper character (Maya) and upgrade her rifle skills. The sooner she can use that rifle at close range, the better her chances.

Use Gennady and his scanner in later missions. It becomes crucial to know in advance where the bad guys are hiding.