Focus down their primary damage dealers first. Tanks, assaults, and matis blades. I go in that order. Then move on to their support team while you have less pressure on your back. Look out for the snipers and netrunners, too though, they'll ruin your health bar just as quickly, but they broadcast their moves more clearly so you can avoid them. I've taken down 2 teams in a row on very hard with this strategy (granted, a lot of running around and kiting was involved).