When you put down a minefield for enemies coming from a known direction, preregister your mortars behind the minefield. When the hit the minefield, open up with the mortars. The enemies can either go forward though the mines or backwards through the mortar fire. If they don't move, have your forward observer start sniping at them.
(to not tip this off, fire some mortar rounds into the area and then lay the minefield, so you have mines mixed with shell craters)
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