[Trigger] Angel Arena Creep Spawn

[imsoimba]

I am currently working on my first map (super noob, just figuring it all out still) but i have all my heroes created and am upto creating the creep spawn, i want it to be like angel arena's but just cant figure it out, what i have so far is:

Creep Spawn
Events
Time - Every 20.00 seconds of game time
Conditions
Actions
Unit - Create 1 Footman for Neutral Hostile at (Center of Creep Spawn 1 <gen>) facing Default building facing degrees

but then i dont know where to go from there, i want it so that after 40 sec it will create i different unit to that 1 then at 60 sec another different unit and then at 80 sec go back to the original footman.

ty in advance to anyone that can help me.

 

[KhaosMachine]

Check my system:
http://www.hiveworkshop.com/forums/attachments/triggers-scripts-269/115753d1342558976-random-attack-wave-randomspawnsystem.w3x

 

[imsoimba]

this system seems to be random can you tell me how to make it into a select unit to spawn rather than random?