Go into to his garage and fix the electrical panel that operates the hoist. Step back towards the middle of the big door. There is an opening between two of the crates. Using your grenade launcher in first person targeting, send in a grenade. If the Thing turns its head toward you, then you have him targeted. Continue sending in grenades until he starts burning. You can send in a few more if desired for extra fun. Run into the Thing's room and on the far wall from the big door is a switch. Throw the switch and watch as the Thing dies. You will probably take one hit from him.
When you first get in the large door, use the previously described trick to bring the medic in. Use your sniper rifle to shoot it to get its attention, then shoot at it for awhile with any of the guns. After that, get between the crack of the two crates near the winch power box and enter first person mode. Aim at the ground with the flamethrower and the Thing will catch on fire. Make sure to have lots of flamethrower ammo to do this.
Use all 25 of your grenades from behind the crates to take it out.
Equip your flamethrower and activate the switch with the blue spark, then move back slightly and face the Thing. If it does not see you, but you see it, start to shoot flames at it. This will take some time since you have to get to a far enough distance so that the flame can reach the big Thing.
When you enter the big room from the tunnels, kill all the creatures in that room. When you have the room secured, save the game. Turn around to the door that leads to the tunnels. To the left of that door is a group of boxes. Behind those boxes, you will see orange grenades. However, you cannot get back there. Walk back a safe distance and shoot one of the grenades. This will blow one of the boxes close, so you can get behind there. You will also find a grenade launcher. Use it to defeat the Boss and not lose any life in the process. Make sure you are fully stocked with grenades, mainly the high explosive ones, but you will need all you can get to defeat him. If you need more grenades, run back into the tunnel to the end. Save the game again, then go into the room where you have to electrocute the Boss. When you enter that room, there is a row of boxes. Walk towards the end where the broken junction box is located. You will see a gap in the two rows. Take out the grenade launcher and line up your shot to hit the creature between the rows of boxes. You should see his top left shoulder. If not, shoot him or electrocute him once, then run back behind the boxes. The creature cannot touch while behind the boxes. You must hit him on the shoulder for this to work. Fire one grenade at a time and be careful not to blow yourself up. If this is done correctly, you will kill him with no loss of life.