Impossible Scenarios type maps are very micro-intensive. The key words are "teamwork", "patience" and "micro".
Basically, you only have a few units (at most 5, most of the times) to defeat a huge army of enemies (most of the time static). This creates very interesting puzzles where the players must think of the best way to apprehend the army and counter its forces. It is also a very interesting kind of map to create since it's really pushing the map-makers knowledge to its limit, trying to find innovative ways to present weird unit combos supposed to cooperate perfectly together.
Here is a little tutorial on how to generally create an Impossible Scenarios map:
Basics to create an Impossible Scenarios map
Because I'd love to enjoy playing some more Impossible Scenario maps on StarCraft 2, I thought creating a little tutorial on the basics that make an Imposs map successful might help. First of all, you must be aware that I do not declare this as being the best way to design awesome Imposs map: I am mostly depicting how I see the making of this kind of maps.
1) Selection system:
- Set 1 beacon per level.
- Try to set a unit representing the level behind the beacons: this will help players recognize levels. Those representing-units should be owned by the Neutral player.
It is important to not force the players to play the levels in the same order over and over again because I swear they will end up being bored to play this 1st level every time they host your map.
- It is recommended to give a rectangular shape to your scenarios since it makes it much easier to fit them with a Region, which is quite handful for the triggering part.
3) The levels (scenarios):
- The very first thing you must start working with is the Terrain layer: it determines the general path the players will have to use and increases the strategy required to beat the level, which makes it more interesting. Indeed, cliffs are a very important aspect of the Imposs maps: you must place them with care.
- The second layer you want to work with is the Unit one. I recommend always starting by placing the enemy's Tanks first since they play a huge role in the overall gameplay of the Imposs maps. The second thing I like to place are the defencive buildings. From there, place the rest of the units around.
- I always finish with the Player's units since it permits me to determine what kind of strategy they'll use to beat the level.
4) General professionalism/aestheticism points:
- Use the grid when placing Beacons, Neutral player units, and the Players' units. Place only the enemy units without the grid.
- Set the orientation of the Players' and Neutral's units so they are facing a relevant point: Neutral's toward the center of the Selection area, Players' facing the level they are about to start.
- If you are using the Orbital to allow players to scan, set the Hotkeys for them.
- Center the screen properly on the players' units.
- Make any units in the Selection areas invincible as well as the Orbitals.
- Edit the Orbitals' abilities to let only the Scan one: the others are irrelevant and may distract the player.
- Share any relevant information in relation with a level only when this level starts. Keeping things clear and short always helps.
- Doodads may or may not be appreciated: it depends on the kind of levels you are setting. I prefer creating big levels, which always lets a lot of hole that I can fill with doodads. If you prefer the smaller levels, which requires more strategy, it is recommended not to place too many doodads to not distract the players' attention from the level itself.
- Use the Hostile player (Player 15) as the enemy: it has a great little default AI.
As for the triggers, here's an example map: http://www.staredit.net/starcraft2maps/34-Impossible-Probe