I'd suggest your DK Tanks should be using Grip and Chains of Ice on the Sappers after they've dropped to the deck to help slow them down, and the ranged should swap to them to help burn them down. But make sure your DK's don't overlap their abilities on the same Sapper at the same time.
#showtooltip Death Grip
/tar Twilight Sapper
/cast [@focus,exists] Death Grip
/cast [@focus,exists] Chains of Ice
As the only melee dps, your cleaving the adds is a very big portion of the dmg they'll take, so if they can grip the sappers to the adds where you can cleave onto them, you can increase your dmg time on both of them, allowing them both to die quicker.
Really helped getting the sappers down with this macro Thank you.