Im no pro at this game so other veterans may tell you something different and I hope they correct me if I am wrong but with Iris seeing as her skill changes after CC and if you wish to use your second Iris as fodder for your main like I do you will need to CC her also (which can be a little costly in terms of silver units if you have multiple, I have had 3 Iris I have used this way.)
I lvled the fodder Iris to lvl 30 and then CCed and lvled once again to 20-30 so my fodder Iris was 30CC30, a lvl 30cc20 Iris will net you about 710 xp so best used with gold spirits too if your main can still be lvled to max. Just remember a CR and skill up aren't guaranteed the closer you get to max CR and Skill up so it can quite a painful method to do if it doesn't always work.
You can find charts on chances of skill and and CR here.