I haven't tried it yet but you might be able to make something work using lists and the pick action. Make the index of the list equal to the wave, so list[n] = wave n spawn.

Pseudo:
list1 = [entity/party A, entity/party B, etc]
list2 = [] (empty; used for temp storage / variable)
trigger: on wave spawn triggers
conditions: optional
actions: pick from list1 using wave# and store (set, not add) in temp list2, spawn unit in list2), wave#+1

Note: You cannot spawn all units in a list by selecting the list as the spawning entity. MightyScript will only spawn the first entity in the list. I've reported it as a bug several months ago. Who knows if it will ever be fixed.


If that doesn't work, then use a manual trigger of some number (ie. manual trigger 100). Then make a spawning script that triggers on 100. Add in a condition that checks the wave number (ie. wave = 1). In the action, spawn the units and increase the wave. Then clone that script a bunch and change the condition # and spawn units.

Pseudo:
on trigger 100, if wave = 1, spawn A, wave+1
on trigger 100, if wave = 2, spawn B, wave+1
on trigger 100, if wave = 3, spawn C, wave+1