Simply burn Sjonnir down to 40%, then stop DPS'ing. Then have the tank kite him around the room while Malformed Ooze spawn from the north and south sides of the room. Malformed Oozes will eventually combine to become Iron Sludges. Kill 5 of these, then kill Sjonnir to get this achievement. If you DPS Sjonnir below 40% before turning your attention to the Oozes, it is possible that a contingent of friendly NPC dwarves will kill Sjonnir before you have had time to kill the required 5 Oozes.
Iron Sludge causes considerably more damage than the Malformed Ooze, or the previous Trogs and Iron Dwarves. The healer should be aware that the party will be taking considerably more damage at this point of the achievement attempt.