For the start: Drop slightly to the right. This should land on the enemy, stun them, and push them left. Kill that enemy first to gain Killing Spree, then turn around for the other guy to 1 shot him (otherwise it takes an extra shot). After the keycard door, take a second shot to make sure the big guy in the basement can't blow himself (even if he doesn't try). If he blows up after sniper the boss-guy, it will make him walk away instead of up the stairs to see you.

Always blow your breach charges when you aren't near them. If a suicide runner corpse happens to be there, it will blow up and kill you when you think you're at a safe distance.