The "Rescue the Scout" shadow mission has 17 waves of enemies occurring at 1 minute intervals.
The problem is that if you are in a reasonably powerful party, then it can take about 5 seconds to kill all the enemies in a wave, and then you have to wait 55 seconds for the next wave. A game that forces you to spend most of your time waiting is not fun. If you provide a method to accelerate the spawn rate so that we can deal with the 17 waves more quickly and complete the mission faster, then we can spend more time on the parts of the game that are fun.
Suggested solution: Add a switch in the middle of the playing area. While there are enemies alive, the switch is inactive. When all the enemies in the current wave are dead, the switch lights up, and then hitting the switch makes the next wave spawn immediately. If no one hits the switch, because say they're healing the 3 NPCs or collecting drops, then the next wave spawns 1 minute after the previous spawn, as occurs currently. That is, if your party is playing near the limits of their ability, they can still take over 17 minutes to complete the mission, but if you are in a strong party, you can get it over with in a couple of minutes.
Of course, this might now mean that a couple of strong humans might be able to knock off all 17 waves of enemies in a single spell of Final Hit. Personally, I don't have a big problem with that, but if the game designers want to avoid that, they could modify the above suggestion to also include a minimum period of say 15 seconds between waves. But I think a better solution would be to improve the enemies' AI. Currently the enemies are all pretty stupid when it comes to reacting to what players are doing. If the enemies see their colleagues being wiped out by a player using Final Hit, any sensible enemy would load counterattack. (OK - not all the enemies in this mission have counterattack, but enough do to make a difference.)
I've made this suggestion with respect to "Rescue the Scout" since that's an easy one to explain. But if you do implement this suggestion, you can then also think about adding it to the other shadow mission that can have tedious periods of waiting if you are in a strong party: Taillteann Defensive Battle. It's a little messier to implement there, because you have enemies spawning at 3 different gates, so you need 3 different switches operating independently. That is, if the pair operating at the South Gate decide to hit their switch to bring forward the next spawn, it should NOT also immediately spawn a new wave of mobs for someone struggling at the East gate. There's also the issue that if the players at one gate fail to keep up with their spawns, the spawn location then moves to a new position nearer the centre of town. So then you need to decide whether to move their switch to the new spawn location. My opinion: It would be nice if the switch moved, but if they've failed to keep up with the spawns at the first location, they are probably not going to want to accelerate the spawns at the next location, so providing a switch that doesn't move is not a big problem. It would still be an improvement over the current scenario of no switch at all.
Anyway, I suggest implementing the suggestion for the simpler case of Rescue the Scout first, and making sure that works properly, before worrying about how to fix Taillteann Defensive Battle.