The mimeflora ?
You need lots of pain...
The first thing : they will each make a copy of one of your units, except that their base stats will be equal to the base stat of you units plus the stats of its troops : they will hit a lot harder (but have not coordinated attacks). So it is best to bring troops with a strong effect and a not so high stats (samachrul, or if you can't, specters) and avoid at all cost troops that are big stat sticks (flowers).
Then, for the shield, each time you hit it, it will lost 1 single point, no matter how strong (or weak) the attack is. This makes AoE attacksvery valuables (Xianyun was extremely useful for me for AoE, and Alice for very long range attacks).
I have a doubt, but I think nycto doesn't work on their shield.
Once their shield is down, they will go back to their default form, which is weak, and you can (finally) make them lose HP. However, you should kill them very fast, as they will retransform (with a brand new shield) after 1-2 turn.
The fight is VERY hard, I would rate it the hardest in the game by a lot. A strange side-effectof the first paragraph is that bringing you bigger strongest unit is not neccessarely a good idea : you should focuse on the mecanic of the fight first, and adjust your lineup for the fight : it they keep copying the same unit and one-shotting all you untis with the copy... remove that unit.
If you want my lineup for that fight, you can ask, but it was not necessarely optimal.