camcreate two shells at the same location and they will explode like flak.
Like this:
"shell105" camcreate [6000,6000,250]
"shell105" camcreate [6000,6000,250]
This will create flak at 6000,6000 on the map 250 meters in the air.
You can also place gamelogics on the ground and use them as locations from your flak.
Like this:
"shell105" camcreate [getpos gl select 0,getpos gl select 1,250]
"shell105" camcreate [getpos gl select 0,getpos gl select 1,250]
This will create flak 250 meters above the gamelogic unit named "gl"