Making a mortar is pretty simple.
1.Make a large map.
2.Place an entity named “env_mortar_controller” and name it mortar_controller.This will be the controller.
3.Place an entity named “env_mortar_launcher” somewhere outside the map and edit the properties to your like (and name it to mortar_launcher)
4.Place an entity named “point_camera” above the map and edit the options to the following:
Pitch Yaw Roll - 90 90 0
Screen rt Camera Ratio - _rt_Camera
Name:mortar_camera
FOV - 60
Screen Aspect Ratio - No
Fog Enable - No
Fog Color - 0 0 0
Fog start - 2048
Fog End - 4096
Fog Max Density [0…1] - 1
(basicilly the options you get when placing it, but theres only a changed FOV 
)
5.Place an entity named info_camera_link and place in the Entity Whose Material Uses field _rt_camera your mortar name (in this case mortar_controller)
In camera name use mortar_camera.
6.Go to env_mortar_controller and fill in the Mortar Launcher field mortar_launcher.
Example .VMF can be downloaded here
Feel free to ask questions, since this is my first tutorial and its probably not really good to follow.
Have fun 
.
Fun extras
Killing a garg using a mortar
1.Place a npc_gargantua in your map and name it “garg”
2.Go to outputs and add the following:
My output named - OnDamaged
Targets entities named - “the name of the gargantua”
Via this input - GargantuaExplode
Add another output, and fill in the following:
My output named - “OnWishToDie”
Targets entities named - “test_garg”
Via this input - “GargantuaExplode”
Example .VMF can be downloaded here
Using a Xen Launch pad as trampoline for the grenade
1.Place an env_xen_pushpad
2.Place an info_target and name it “pushpad”
3.Go to the env_xen_pushpad and enter in the Jump target name the name of the info_target you placed.
4.Edit the height offset to your prefer.