Cheese comp is a strategy where you use a shield tank such as orisa or rein protecting a torbjorn turret and a bastion. The other three hero slots go to mercy so she can revive a fallen teammate , symmetra so she can use her turrets and proton barrier and a character of your choice.
This strategy should only be used on point capture maps.
Most teams lose to cheese strategy because they:
1) Do not take the time to learn where fortifications are prior to attacking. Where is torb turret? Where is bastion at? Where is symmetra teleporter/shield? Where is sentries? Where is junkrat at? Therefore, randomly dying to surprise.
2) You lost your healer and did not kill anyone... Why are you still trying to attack and put yourself at risk?
3) Two people died and didn't kill anyone on enemy team. Why are you trying to press an attack at all? Wait for full 6v6 again.
4) X character used ult while half her team was dead and there is no one to combo off her ult or nearby to take advantage of it... Dumb move. Save ultimates when you legit have an advantage you can take off it.
5) People fail to take the initiative when they legit have the advantage. Your team is 5v3. Take that advantage and go in.
Basic Sombra rhythm works even with a non cooperating team. Essentially, you keep harassing them teleporting, heal, invisible, harass, teleport, heal, invisible, etc. to build up your EMP and keep ulting them until your team finally responds and moves in while everyone's chasing you around after the latest round of EMPs. Sombra can do it consistently and continuously for the entire game so it's just EMP, EMP and more EMP. It works in the lower levels, so i assume it works in the upper ones as well.