Here is how to get out of the difficult level 9 if you're playing the Atreides family. Use a Hex editor to edit the SCENARIO.PAK file in the Dune II directory and do the following:

Save the level 9 game.
Go to scenario 22 of the Atreides.
Go to [ATREIDES] and change credit to 30000 and Maxunit to 50
Go to [UNITS] and change all Atreides Siege tanks to Atreides Sonic tanks.
Go to [STRUCTURES] search for palaces of the Harkonnen and Sarduakars and change them all to Ordos palaces.
Go to [REINFORCEMENTS] and change the Atreides tanks to Sonics.
Save the new SCENARIO.PAK file. Start the game and load the saved level 9 position.
Go to OPTIONS and RESTART SCENARIO.

You now have loads of credits at your disposal and all of your tanks have changed into the more powerful Sonic variety. What's more the missile barrages should have stopped making the chances of you surviving the level far higher.

Place concrete slabs before placing structures. It's worth upgrading the construction yard so you can build the larger slabs. Upgrade the light vehicle and heavy vehicle factories as soon as you can so you can produce siege tanks. It's worth upgrading the heavy factory several times on later levels so you can produce a mobile construction vehicle. Watch the state of all your structures and repair them when necessary - it costs spice but it's usually cheaper than building replacements.

Don't bother building walls (they cost too much and don't last very long) build rocket turrets instead. You need to upgrade the construction yard to be able to build them but you're going to do that anyway aren't you? Spot where the enemy are attacking from - they always attack from the same direction - and place concrete slabs on the rock in that direction. Put half a dozen turrets on the extremities of the slabs and with a bit of luck they should see off most of the attackers before they get too close. Keep an eye on the state of the turrets and repair them when necessary.

See whether there's a line of rock that stretches from your base towards the enemy base. If there is create a pathway of concrete slabs along the rock and position rocket turrets close to the enemy base. When you attack the turrets are able to give your units covering fire and you find yourself with a significant advantage. If there isn't an unbroken line of rock consider using a mobile construction vehicle to establish a construction yard on a piece of rock close to the enemy which you can then use as a base for extending the range of your turrets.

On the later levels it's extremely important to restrict the amount of spice the enemy mines because spice runs out extremely quickly. You need to use a fast trike or quad to explore the map and find spice fields and enemy harvesters and then you simply do everything possible to destroy the harvesters.

If you want to use/build more than 25 units use a Hex editor to edit the scenario.pak file. Find a line containing:

Brain=Human
Maxunit=25
Credit=1500

... then change the number after the Maxunit to 99

Scenario.pak contains all the info about structures and units at the start of the level, so have a fiddle and give yourself some better units. I haven't worked out how to place units accurately on the map but from what I can tell, if you add one to the last number of the unit or structure it moves one square to the right (for structures the number represents the top left corner). I have discovered that overdoing this hack can cause the game to crash when you try to restart the scenario, and editing Level 1 just doesn't work at all, however you can still have some fun, and give house-specific units (e.g. Devastator, Sonic Tank) to other houses (just remember that no matter what affiliation a Deviator Tank may be, it still turns units to Ordos, which is not very good for the game if there is no Ordos player. I don't know if it will crash or not, as I have avoided doing this).

This cheat involves editing one of the Dune II files, so remember to back up before you start fiddling around.

Edit the file scenario.pak with a text editor such as write for Windows (Do not use Edit in DOS! This damages the file when you save it so don't try it). This file contains all sorts of useful information about each of the scenarios for the different houses. There are 21 scenarios for the different houses and I believe they work as follows:

Under each level there is a map seed (do not change this unless you want to start in the middle of some sand) and some info about the players. Find the scenario (or scenarios) that is/are applicable to the level you are on and change the credits for the human player to -1. Save the file and reload Dune II. Load your game, then restart the level. This will give you 65535 credits, although it looks like you have -1 at the start (the credits will go down to -32767 then flip to positive 32767). Play on and enjoy your cash.

As you may have experienced, controlling your units its very clumsy with the mouse alone and it can take frustrating long to manoeuvre units around the map. To speed up your actions, simply press M to move or A to attack on the keyboard and leave your right hand free to click on destinations or enemy units.

This is more of a typical "It's not a bug, it's a feature" type of thing. The first one doesn't occur always, but I managed to win a level with it once.
Whenever your Harvester returns to your refinery with an Carry-all it seem to refine some spice, almost nothing, but mostly you get 1 fund.

A cool trick, only for the Ordos. Simply follow these steps...

Too bad this will only last untill the AI gives a new command to the unit, but it is possible to destroy nice things like the house of XI or even palaces ;-)

To elminate enemy Harvesters, attack them with infantry. The Harvester will do its best to crush them under its treads. Allow this to happen. The harvester is now stationary and will remain so forever... ;)

Keep screen focus on units you want to move faster (like your own), and keep the screen away from units you want to move slower (like your enemies). Build a wall of turrets to eliminate incoming enemy units, but don't watch them get blown up! They'll move so slowly that they'll be more likely to die.

The AI is dumb. It has a high priority to attack certain structures and ignore others – heavy factories, repair facilities, and palaces are some of their top priorities. Build your base accordingly, to direct enemy attacks in a particular direction, and build a wall of turrets between the enemy base and the highest priority targets you have at your base.

As the Atreides, once you can build sonic tanks, that's all you need. Forget every other unit, and just build an army of sonic tanks. Bunch them up together and move them slowly – they'll be an unstoppable force, since they have long range, can't hurt each other, and can do damage to multiple enemies at once along a line.