Seeing the goblins go into cover, David shouts in Goblin, "What? Too scared to attack? Well looks like I was right about your mothers wasn't I?" Then he'll just take a potshot at one of the goblins behind the rocks. (OOC question. Don't the goblins just have a bonus to their AC? Like either a +2 for half cover and +5 for three quarters? if not, understood)
Attack: 7 Damage: 6
((if not at disadvantage, attack would have been a 19))
Throin looks around for a way to get to the rider quickly. He spies the horses tied up near the fire, and runs and jumps on one. (https://5thsrd.org/combat/mounted_combat/) Mounted, he rides between the others to get to the front line (using horse's movement and Dash action). When he gets within range, he will Attack with his warhammer.
Throin Movement: 10 feet to horse + 15 ft to mount
Horse Action: Dash
Horse Movement: Up to 120 ft to arrive beside rider
Kethax will swing his greatclub perpendicular to the ground hoping to crush the goblin's head in front of him, and then will then continue the spin smashing the goblin with his tail. At which point he will start to crouch and move around the rock moving on to the next Goblin.
The goblin rolls iuder your great-club hoping to catch you from behind finds himself lined up for a perfect tail attack Kethax's tail whips around catching the goblin almost at the end of his roll smacking him in the head breaking its neck. Kenthaz moves behind the rock searching for other goblins