The previous one will cause the AI to use it at the first opportunity. The question is how to restrict the AI to use it only after a certain time period spent in combat.
The tennis interaction method will cause creatures to level up while spending time in battle. Combat hardness is a little different, it's an internal value that rises as a creature 'becomes accustomed to tragedy'. This usually happens by watching their friends die, but it can also happen if they survive major injuries or trauma themselves. (Not sure how it would work in Adventure Mode though, but tennis interactions are also kind of broken in Adventure Mode anyway, so...)
It depends on how you want to do it, but using combat hardness is a much, much simpler and cleaner method. Even better, you can easily do multi-stage evolution, something which is virtually impossible through other methods. Simply do the following:
Create an interaction like this and put it in the interaction file:
[CE:CREATURE:SUPERMODE:DEFAULT] <--CREATURE:CASTE as before
[CE:COUNTER_TRIGGER:COMBATHARDNESS:33:66:REQUIRED] <--Adjust the start/end levels as you see fit, COMBATHARDNESS is a value from 1-100
This syndrome will cause the creature to evolve at the given levels of combat hardness. However, it must get the syndrome in the first place. To do that, give it the following interaction in the creature file:
[CDI:ADV_NAME:Become an evolver] <--If you take this part out, it will still work in Fortress mode, but you won't be able to do it in Adventure.
[CDI:WAIT_PERIOD:33600] <--Number is irrelevant, but I think you have to do it or the creature will try to use it at random later.
And it's finished! (Note: I haven't actually tested this yet, but I have done things much like it so it should work.)