How to beat this???

[Flinger]

Why should it be illegal?
Has 3 defenders, 2 players in each wing, has a ST and has 5 players on attacking side of the pitch... Only reason for being illegal is players out of position.
That said, i'd probably go with a 451 (more a 4 3 2 1) or a 442, attacking down the flanks.

It really depends on the orders he sets, so it's a wild guess.

A weird ideia is going for an asymmetric formation yourself. Like:
-----ST-ST
---------------AMR
ML MC MC MC
---------------DMR
---DC-DC-DC

No idea if this would work