SPOILER
just tell them to, with an 'area target'
best thing to do though, is use the striker that is designated as "FSV" to call in a linear artillery barrage on that trench, from one side to the other
you could spend all day plinking at them in that trench, and all you will manage to do is suppress them.
(This is probably why they arent firing, they know that the chances of killing are slim. They will automatically fire at any exposed infantry, outside of a trench, or that are firing from a building)
also, you could pull up one of the strikers that has the 40mm grenade launcher, get a 45 deg or 90 deg angle up on the trench, and then start dumping in bombs,
hope this helps