The tactics to 5-aside are simple imo.
Never let a runner go free. When the opposition have the ball stick with your man at all times.
Avoid getting caught short at the back at any cost.
Shoot whenever the chance arises.
Tons of movement both on and off the ball is essential (applies to attack and defence).
I'm not great at football by any means (not too bad though), but my workrate makes me a pretty reasonable 5-aside player. We've played loads of games over the years where the other team has far more possession and have totally dominated the game. But if you stay close to your man and stop them getting clean shots away, then it is more than possible to win the game. Work hard as a team to break up the play, and when you do win the ball, use quick passes and break with pace.
General formation imo should be:
Attack - 2 up front with 1 defender supporting and 1 defender deeper, offering an out ball.
Defence - 2 and 2 in as close to a box as you can. Stick with your man if he makes a run. If they play with a deep defender (most will) you should have a free man. Use him to press the deep pass, he's not there to win the ball, just to make the pass harder for the playmaker at the back. The guy pressing the ball must not dive-in, if the guy on the ball goes past him, then you're already a man short at the back.