Attacker

[Samkun]

How to make AI units to attack me / my allies continuously but with few troops (like warcraft 3 : reign of chaos, orc campaign -> cry of the warsong), ?? (not spawn but the ai train the units)

 

[Rogue Kessler]

There's a simple way and a more complex way, depending on how much you want to manage the AI.

Option 1 - Use triggers to order specific AI buildings to train specific units (like Grunts from the Barracks), then use triggers to detect those troops being trained and order them to attack-move to your base. You can set these up as a loop so they keep firing, as long as the AI doesn't run out of gold.

Option 2 - Use the AI Editor inside the map editor (the icon with the picture of a green face on it, to the right of the Campaign Editor). There you can set up everything for the AI, from the order it builds its units to the size and makeup of its attack waves.

 

[Daffa]

For campaigns, it's recommended to use JASS AI to make them more intelligent in suicidal attacks.

 

[Nowow]

Option 1 - Use triggers to order specific AI buildings to train specific units (like Grunts from the Barracks), then use triggers to detect those troops being trained and order them to attack-move to your base. You can set these up as a loop so they keep firing, as long as the AI doesn't run out of gold.

This is usually not a good way to go. The result of having units "attack move" somewhere is that once they are aggroed and finish an encounter, they stop.

You can learn to do this with either AI editor or JASS AI like Daffa said. For JASS AI you can start here:
How to make a Campaign AI
Creating AI workflow

Also, yiu can extract the actual AI file of the mission you're talking about and see for yourself how to do it.