These mobs come in pairs, and are generally both tanked. They are immune to stuns, hunter traps, shackle and taunt so let the main tanks get enough aggro before you attack. Make sure to assign dedicated healers to both tanks. When they apply their debuff, these healers (and any unassigned ones) immediately have to overheal the respective tank since the damage caused on him will become murderously high. The Stewards will usually do an emote when they apply this debuff, so stand close enough and watch out for speech bubbles to see your cue for when to start overhealing.