Eventually I noticed that dirges and frost giants are on different factions, so, depending on your faction, the giants will not add when you pull them, even the named.
When killing this named be sure to control (time) the storm giants that wander the area. A giant may eventually add to the fight and omplicate things (if you have a single merc, for example).

Evanescent Coronach's Fiery Tongue
Greater Dreadmote
Lesser Dreadmote
Median Dreadmote
Minor Dreadmote
Shield of Greatness