Hi there,
I'd maintain the formation as far as possible. You can try to experiment and change one or two position every time. Also, if your opponent is weak on the wings, make sure to concentrate on the wings and vice versa.
I've got a question myself:
My next opponent has this as his formation:
3-1-3-2-1.
DL----DC----DR
------DMC-------
ML----MC----MR
---AMC--AMC---
-------ST------
I am Kinda helpless since his players are better than mine for the most parts and he has an UBER-strong middlefield and still got a lot of punch with two AMCs and one ST. I guess his DMC will work as a second DC moving backwards so he has that covered as well... Please help!