Make a CAS, Mortar, and Support Sqad
1 Men in Field as Sniper who Spott
1 Men Who Reload the Mortars and Defend the fob (most times From the AA)
2 Men in the Mortars who fire
2 Men who Drive or fly to deliver Supply Crates Or fly the Atack Helo if its available
and a Commander who Spotts if SNiper cant see target
And then it should work so.
Build Mortars (so that enemy has problems to Spot them but even so that Support Crates can be Delivered Fast and without danger)
With a Distance of 10-20 Meters (hold longer by enemy Mortar shootings and bether Spread.)
And then Work in the Following steps
Step 1:
Commander or Sniper give target. (Sqadleader (Sniper) set Atack marker on this Position
Step 2:
The Mortarshooter 1 Read the Distanze from the map (its exactly enough) and estimate the High Difference . He Calculate the Angel (with a Difference of 20 Meters away from the own Troop Positions (so that the mortars shoot not accidentially on own Troops).
Step 3:
Mortarshooter 2 Use the Angle Shooter 1 Has Calculated
Step 4:
Shooter 1 fires one round high explosive
Shooter 2 counts up to 7-15 and fires then his round
Step 5:
Spotter and/or Commander watch for impackt and give feedback. Because of the 15 Sekonds delay they know which impact is caused by which Shooter.
Step 6:
Both Mortars adjust their Distanze so near as possible (if no own Troops near 50 Meters to near.)
Step 7: Request for Full fire. (Commander and Sniper check that theres no change at Target area last (1-2) Minutes since Step 1
Step 8 If Fire Command is Given
Mortar 1 Fires all 10 Rounds he got but after each Shoot he press W or S shortly and/or A or D Shortly
Mortar 2 does the same but in the other direktion (along the Front)
So you will have an ideal Spread and really heavy impact
(Imagine it like you Paint a X with the mortars, or maybe a II ,(dont try to make an O its hard to Explain and most time useless

) Or both Shoot an I but with different ammo types.
If you have the Commander support he can watch the behavior of the enemy after the first 2 Singel Shoots.
If enemy go in cover maybe make a Short Break after Step 6.
Step 9:
Both Shooter go out and put 1-2 Ammo Bags inside the Mortars
(The rest makes the Reloading guy)
Back to Step 1:
(the rest of reload could be done so long the mortars going Trough Step 1-7
If Cobra is Activ the mortars could be Reloadet by the Shooters and the Reloader does the Resupply Part.)
I have tried this (without Cobra) and it worked superius.