I have a game vs flat 4-5-1 and i will play flat 3-5-2 like that:

ST ST
ML MC MC(up) MC MR
DC DC DC

Defensive mentality and counter. Mixed passing and attacking every where.
Maybe put down arrows on ML&MR if your opponent has good player.

If the opponent is very strong i d play: 3-2-3-2
ST ST
MC MC(down) MC
DML(UP) DMR (down)
DC DC(down) DC