Let's think about things tactically:
DEFENCE: The drawback of a 4-3-1-2 is that it has no inherent width except for what the full-backs can provide.
Playing 3 central defenders at the back (two in man, one zonal OR all zonal) will ensure you outnumber your opponent's front line with a spare man at the back. With no wingers to face, you should mop them up as long as you don't play too high up.
DEFENSIVE MID: Play an anchorman just in front of that 3 - you can choose to man-mark the opposing playmaker to nullify him. If you fear runners from midfield, play 2 men here; with the other in something of a deep-lying playmaker role with direct passing instructions.
MIDFIELD: With essentially 4 men in the center of the park as a 4-3-1-2, you're going to have a torrid time trying to pass through the middle. Instead, go with two wide players playing a wide midfielder role. Make sure they can get ahead often enough.
FORWARDS: Lastly, play 3 up front in a wide trident. Use your wingers as wingers (as opposed to inside forwards) and close down your opposing full backs. Pin them deep on the flanks. Between your forward wingers and your wide-midfielders, you'll overload the full-backs while also drawing the midfield trio very wide.
Controlling the game is difficult with so many players in the middle of the park - instead look to quick, wide and direct play to find a way to score whilst maintaining a reasonable amount of possession.
Good luck!
PS: Alternatively, you can match the 4-3-1-2 with your own and hope for a scrappy goal. OR perhaps just get very narrow and boring.