Im playing level 10 with Belper Town and found that shield wall kept conceding goals when hanging onto a lead.
I adapted conqueror to a defensive/ time wasting formation, and found that my team kept the ball really well and more often than not grabbed another goal, if im winning comfortably i tend to use this for the last 10 minutes.
Hopefully it may work for other people.
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If you want kill the match then you'd better go for "The Waste Time Tactic" which can be found at the OP.
I don't think that Shield Wall is the best option if you want to kill the match