Quality of both teams are roughly around even so thats not a issue since both sides would of had a good chance of a draw had his formation would of been legal and possibly your opponent only having 20% possession quite easily could of played counter attacks, If you had a good look at his setup before the game, it probably would of been worth attacking his right-hand side where he only had 1 player present, since his left and central looked packed with players, attacking the right side only would of been exploiting his weakness and you would of probably got more success scoring goals. Whenever I play teams that use an illegal formation, I tend to go all out attack since I know I am going to have 80% possession and I want to make the extra possession count.