It's actually really simple.
The Epic Armageddon rules wrote:
all you need to do is bend over and get a ’model’s eye view’ to see if they are in each others line of fire.
Just bend over and look from the weapons/stands PoV.
If you can see the target clearly, shoot with no modifier.
If any part of the target is partially obscured, apply the -1 modifier.
If the target is not visible, the shot cannot be taken.
The Epic Armageddon rules wrote:
The method used by the author (and the default you should use if you can’t agree to an alternative) is that a weapon is in range if any bit of the attacking weapon is within range of any part of the target model (or at least one of the models on a target stand).
Whilst this is spcifically mentioning weapon range, it is clear that LoS and range should both be measured from the weapon.
This also works easily with area terrain as long as the area terrain is defined clearly (ie, Los is blocked to the edge of the area terrain and to the height of the highest terrain piece - be it trees/buildings etc).
Anything else is trying to complicate a situation that is really simple.
Again though, the 5 minute warm up is the place to make sure both/all players are on the same page.
There are good points to TLOS (though very few in a game with no fixed scale) but simplicity really isn't one of them