Illegal? Why?
From Justdomes guide:
5.1.1 Illegal formations.
...
In order for a formation to be legal it must have:
-At least 3 defenders.
-At least 1 striker.
-At least a player in each flank.
-A goalkeeper.
So not illegal at all...
He wont have the ball at all, will play defensive or hard defensive, counter and long passes... That will be really good for you. But he may have some arrows in the DM and mixed with counter (to be more attack effective). You will see this on the game itself, if he has decent possession, he will have arrows, if he has no possession, no arrows.
Formation, many hard attacking ones will work. I think 3-1-1-3-2:
---ST---ST
AML-AMC-AMR
-----MC
-----DMC
--DC-DC-DC
Can be good, if you feel ur DMC is useless as his AMC and striker are nullified, move it to MC to win more possession. But wait for someone more experienced than me.
Orders, pressing whole pich will give you advantage, as he will always be playing in his own side, so any ball u get there may be a goal chance, and will ensure the action to happen more in his field than in yours. Mixed passing as only short wont work well with the formation and few gaps for short passes will be found on his defense. Attacking or hard attacking mentality (depends lvls thou). Passing both flanks or mixed (maybe is not a good idea to limit this as he is strong everywhere). Zonal marking. Aside from that, offside trap may be a good idea if ur def are fast. Counter, dont think is a good idea, as with that formation, counters wont be much effective.
Gl with it!