I’ll answer each of your questions in numbers since they are bulleted.
Ideally you want to keep a medium range, ideal so that you can use the shotgun for headshot damage and get the most out of it. Medium range is also good for mortars because depending on how far you are from the monster the mortars drop at different speeds. Too far away- The monster has longer time to react to what he hears and can dodge them, that and it makes it harder to predict the monsters movement if you are far away.
Tell your friends to stand on cliffs, when the monster focuses them shoot the mortars directly at their feet so that you get instant mortars. Also remember that if you are below something like a platform that mortars go through objects.
You want to be at a medium range again because at this range (10-20 meters away) you can hit all bullets on one shrapnel grenade, or the spread will hit others on the monster’s body.
Properly aiming mortars when a monster is not fighting requires tons of experience, you have to think where the monster wouldn’t expect you to shoot.
In a battle you of course have to shoot where the monster is focusing, time the barrages correctly so that the monster will tank them possibly while they are doing an ability, catch them off guard.
It’s all about prediction, that’s what makes a good Torvald.
To know how important mortars are to his kit.
If you catch onto the fact that this monster hates mortars and jumps out of them when they are focusing, take note of where they jump to, most likely they will jump to the same place over and over again, so just mortar that area watch them try to dodge, then laugh when they dodge right into them.
Tips on what I call random mortars would have to be depending on where the monster is headed after the dome drops. First you must examine where they are fleeing, is it straight? Can 100m of mortars go right down without hitting debris?
Next line up your cross hairs 50° off the monster in the air, and let them fly.
It’s just random luck If they hit, that’s why I call them random mortars.
The most simple way I do it, is when a monster focuses me in that rare occasion, do a 180 so that the monster is behind you, look the distance that the mortar restriction is, I forget something like 5 meters close to you and mortar the ground, then boost into it. This makes it so that the mortars explode on top of you, if the monster really wants you he can eat some mortars.
I do a combo with Torvald, leading into the fight with Mortars, then I do Shrapnel, then shotgun, repeat.
In 2 turns of this I believe they disappear. Sorry I don’t have an exact time.