After reading how the guards work in that adventure -- (I'm performing the role of Overlord in our campaign group for the first time around by virtue of the fact I've played twice before -- heh.) -- Two things stop your scenario from working the way you've described, luckily enough for the players.
One -- the Terrain. Line of sight doesn't go through walls. The guards begin in the Gatehouse and Cabin. Both have solid walls that do not provide line of sight unless you get into the buildings themselves, so the Goblins you'd be using would have to get onto their tiles, thereby activating the guards when they get close enough to attack. Also, there are no goblins as default in this encounter. There are Elementals, an Open Group (Which I'd put either Barghests or Ettins), and Belthir. At any rate, you have to get onto the guards' tiles to shoot them, unless the heroes touch their tile to activate them. Once an enemy or hero is on their tile, they get to move on the last hero's turn controlled by that hero.
Now, nothing is stopping two of the guards from being sniped in the way you describe. But two of them are inside the buildings and can't be hit without an enemy entering the tile and activating them. If you choose Goblins as your Open Group, they start on the waterfall. They get the terrain modifiers for crossing water, paying 2 movement for crossing the little pond at the bottom of the falls, meaning only two of them would be in shooting range during the Overlord's turn.
Given that, you still have to successfully roll range and enough damage to kill the guard, with one attack. The odds are against you continuing the assault and winning the scenario that way because …
Two -- The Thief. His Heroic Feat is being able to end his turn and essentially teleport four spaces away from his last space. With a double move -- 8 spaces, he can use his feat to appear in the Cabin, activating both guards in that building, if he starts in the upper right entrance space. The guards get a blue and red die for attack, which means they do Act I Ettin level damage and have Ettin level (or a little more) Health. They're also Heroes. Which means the Thief gets their defense die as well as their own when he stands adjacent to them.
So, yeah, goblins would be a nasty choice -- definitely as harassers, but you've got Jain Fairwood whose Feat is to do a double move AND an attack, with ranged. So, she can run 10 squares then shoot one of the tan goblins, reducing the harassers.
Unless they lost the first encounter, the Heroes can hustle to the guard towers to activate them while ignoring Belthir until they've alerted the guards.
Or they can sacrifice two of the guards to (hopefully) whack Belthir in a turn or three with a blitz assault, but I don't know if that's feasible to keep the guards alive as well.
Just saying -- only two of them can be caught unawares with range.