I had a tough time with that one too. Create an oil surface, or a water surface so the edge of it is in the open fence gate and the surface is inside the area where they're sleeping. The character with highest perception I think would have the farthest range for throwing a grenade or casting a spell for it. Rain isn't a good idea because it makes characters wet and slow, also leaving them vulnerable to stun and if you use any fire, you get the smoke, which those goblins are prepared to overcome. With a water surface, make it ice by throwing an ice grenade or shooting an ice arrow or casting an ice spell on it. It'll make it slippery and when they get knocked down, keep stunning, burning, poisoning, and slowing them to ensure they don't get to you. One of those goblins hit me a bunch of times too, and I didn't defeat them. They're also quite prone to blinded, so Razzle Dazzle Grenade works, which is different from a smoke cloud in effect. A fire surface is easily created by combining tossing an oil flask at the ground, or casting Boulder Bash, and setting it on fire with a firestorm grenade (bottle filled with oil + fuse to craft it) or casting Flare/Fireball at it. But if you use fireball, stand back far enough to get damaged by it and burnt.
