Well, I've never done any TD, so I can't teach you as an experienced person... But, that would be pretty easy. Don't you have any unprotected TDs? You can learn from them.
CONDITION:<<<integer comparison>>> (units in playable map area owned of player x) equal to 0 (sry but u have to search for the correct function)
ACTION: turn off this trigger turn on next trigger...
but i made start "wave" if player X becomes supply >0 and if the games starts round 1 and 2 comes at the same time
this means always when supply>0 the trigger fires - and if thats even true in other waves, the one trigger fires too. you have to turn off trigger or set supply to other value or add a second condition combined with AND-function: COND if player X becomes supply < (1 or whatever u have)