"We've Got Hostiles!"
"We've Got Hostiles!" will most likely be the first real challenging level for almost everyone. It's about the size of Unforseen Consequences, and introduces a worthy adversary, the HECU (Hazardous Environmental Combat Unit) Grunts. The HECU Grunts are human footsoldiers who know how to work together, flank, and use a lot of tactics. "We've Got Hostiles!" takes place across several Black Mesa industrial facilities.
As soon as the elevator doors open, exit and turn right. You'll immediately see a scientist pleading for the blast door to be open. However, his plea doesn't end up being taken care of, as the security guard in charge of the door gets dragged away by a zombie. The scientist runs off...directly into a laser mine. You'll notice several of these laser mines throughout the entire stage (the mines give off blue lasers in whatever direction it is facing), and as soon as you step over them, they'll explode.
Next to the large blast door is a health station and the first HEV charger you will see. Charge up your suit until the HEV charger depletes (you'll seriously need it for this stage) and use the health station if you want. Since there's no where else to go, proceed down the hallway where the scientist ran off to. You'll immediately see some red lasers as you turn the corner. These red lasers (don't get them confused with the blue laser mines) will trigger a nearby turret, so avoid stepping over them when you can. Try to jump over the lasers as best you can to get across, and take the MP5 SMG in the right corner, next to the dead soldier. If you trigger the turrets, run to the other side of the room as fast as you can (don't forget to grab the MP5 SMG in the corner) and smash the fire door activation switch.
Whenever you see one of these red firedoors, always smash the nearby activation switch (unless otherwise noted), as it will block many enemies from coming through. In the next passageway, crouch under the laser mine and then jump over the next one. This passage leads to a lounge, and as soon as you walk past the soda machines, two Vortigaunts will spawn in the checker-tiled room. Kill them immediately with your Shotgun and use the nearby health station if you need it.
Proceed down the next passage on the left, and jump over the red laser at the intersection. This room has four turrets inside; two directly in front of you when you enter and two on the other side. A Vortigaunt will then spawn behind you, but it will trigger the nearby turrets, which will gun him down in no time. To destroy the now activated turrets, take out your grenades and throw one between the two. The blast should cause both of them to be disabled. Now walk up to the green boxes on the other side and keep jumping until you reach the top. Before dropping down, take out your shotgun and go to the very right side. Destroy the turret directly beneath you with a well placed double-barrel blast and then jump down. Now run as fast as you can to the fire door and smash the nearby activation switch and jump up on the pile of green boxes in the next room.
Before dropping down, I advise you save it, especially if you have the Source version. Make sure your shotgun is out, and then drop down. Be careful though, because the floors are very slippery and there's a slight chance you'll fall into the elevator shaft. Two turrets wait around the corner, so jump over the trigger lasers as best as you can, and if you activate them, immediately run forward to the two turrets and fire double-barrel blasts at them. When both turrets are down (if the scientists survived, there's no use for them, so just leave 'em there), shoot the laser mine in the next room to destroy the other ones. This explosion may or may not activate the fire door switch, so run into the next room if it does. Collect the pistol clips and batteries in the next room and then climb up the ladder.
Walk across the platform, and before you go up the next ladder, smash the nearby crates for a battery. Now climb up the next ladder and walk across the next platform. Before you enter the next room, you may want to save your game, because you're about to encounter the first HECU Grunt. As soon as you walk in the room, a scientist will run down the flight of stairs. Use this distraction to deliver two double-barrel blasts to the soldier's head, and grab his MP5 if you didn't do so in the beginning. There's also two clips of MP5 ammo in the corner, so don't forget to grab them. Now press the elevator switch and step inside. Save the game as the elevator goes up, and take out your MP5.
When the elevator doors open, a scientist will blindly run forward...not making it very far. Ok, this will be your first major battle against the HECU's. The most important tactic to apply here is to find cover, peek out, fire, and find cover again. If you run down the left passage directly to your left, you can shoot the explosive barrels in the distance to hopefully kill the first grunt. Draw the other two out, find some cover, and apply the tactic listed above. If they thrown any grenades in your direction, run away as far as you can (but make sure you're still behind some type of cover). I recommend the MP5 to take these guys out, but only fire it in short bursts to ensure accuracy. When all three Grunts are down, use the HEV charger and health station way in the right corner. Climb up the stairs and go through the open fire door (you don't need to smash the activation switch). Walk across the platform and into the next hall, avoiding the Barnacles. Avoid the passage on the left, as we'll go there soon.