[Imp]

Depends on distance to close & terrain but air strike 19 turns in I would use the mortar to lay smoke so I can deal with just one or 2 units without worrying about the others. Deal with them pull back slightly if needed due to incoming air.
Any are time I would concentrate mortar fire on the MMG, more chance of a kill as a grouped unit & also dust might obscure LOS.
If possible working a unit round to the rear to pick off encourage routing units would be high in my mind. Mortar & airstrike vs low morale could cause a route with little extra input.
If attacking from the rear looked good at the start might have used time before airstrike to get there & set my route hex so I routed in the same direction as them.