This is a very brief guide through all of the missions. As Tough Terry is a very fast game that's not very complex, there isn't much to say for most of the missions. Basically, these are just hints and tips on how to solve the missions.
For general playing instructions, see the manual.
This path through this mission is pretty obvious. Just follow the path and kill all enemies.
At the end of the mission, you will leave the house and enter an outside territory. There will be enemies to your left and to your right, so you should not walk away from one of the groups as you will probably walk right into the other. Just kill all enemies on the one side first and the ones on the other site afterwards.
This mission is symmetric. There are 4 rooms in this mission, 1 in each corner. The rooms are connected by a path that has the form of a cross. Three enemy groups with 4 enemies each are standing at 3 of the 4 ends of the cross, and groups of enemies will be standing in the room. Just clear one room after the other and you're done.
You will start this mission in a small house. Equip the best weapon you have (using the keys 2 or 3), because outside of the small house, a huge numbers of enemies will be awaiting you. Just hold the fire button down and blow all the enemies to pieces.
After you have done that, move to the entry which lies at the other end of the courtyard. Once in there, be ready to meet the strongest enemy in game for the first time. It's only one single instance, but better be prepared.
Now, move on through the house. At the end, you will get outside again. Enemies will be hiding behind obstacles, so you might have to move around to get all of them. If the mission does not end, follow the way back, you probably forgot an enemy in the winkled room.
This mission is the first mission featuring darkness and the flashlight. The default key for using the flashlight is F. Use the flashlight to make your way through the level. You will only be able to use the pistol while using the flashlight, but don't worry, the level was designed to work like this and it can be done.
Watch out for enemies hiding around corners!
This mission is probably the most innovative mission in the whole game. You are in a maze, equipped with infrared vision goggles. Use them by pressing C (default). Turn around and look for enemies. The brighter they glow, the nearer they are. To get through the maze quick just look where the enemies are. They will guide you through the maze. Watch out, while looking through the infrared googles, you won't be able to attack.
After you got out of the maze, a group of strong enemies will await you. Be prepared!
You will start this mission surrounded by walls. Don't rush out of here, many very strong enemies are waiting for you. Approach them slowly, taking down one after the other.
When you continue, you will get into a temple. Wow, new graphics, sorta amazing! After you went through the temple, you will be in the jungle. One last enemy awaits you here. Welcome to the new world!
If you want a story explanation for the jungle: While walking through the house, the aliens have created an invisible beam field at Tough Terry's location to get him into a territory that offers more advantages to them and less to Terry. That's why they sent him to the tropics.
For the beginning of this mission, the only hints I have for you are: get used to the new graphics, use your infrared goggles wisely and kill everything that moves.
At one point, you will reach a jungle region which is densely covered by plants. It is hard to detect the distance of walls. When you reach this part, the infrared goggles will become your best friend, because the aliens have adapted to the environment and are cloaked. Walk slowly and kill all of the hidden creatures to proceed.
You are on your own now. Good Luck!