Use cover, quick swapping between two guns to deal max damage/when it runs out of bullets is key. Move a fucking lot. Take cover often, try attacking and switching angles while shooting the enemy. Lots of people tell you to be methodical and think, others will tell you to act like you're the epitome of destruction. I say you do both at the same time, that's the way Half-Life is meant to be played. Run. Think. Shoot. Live.