i think it should be a little harder then full heal/cleanse:
after battle each unit is cleansed, then each statistic of each unit is checked separatelly and modified:
- if the stat is above its starting point it gets reseted to the starting point
- if the stat is below the starting point it gets reset to a fixed number of 75% of starting point
that way any damaged statistics is reseted to 75% (which can be additional hardship if you were damaged less then 25%, or it can be a small healing if you were more damaged)
and any overbuffed statistics will get reset
i think its more fun then just plainly starting with same statistics each wave, raises difficulty a little 
i like this very much, with a whole leaderboard added to it i think thats best way to make it happen
also i think all enemy teams being preset by devs is splendid idea, ppl could create whole meta chat about how to defeat wave #24 or #67 i think thats wonderful way to add more skill/team building into the game