That one definitely is a pain to capture. When I captured it for my Necromancer, a friend of mine offered to run me to the Dreadnought's zone, and then fight from there. It's a whole lot quicker than fighting north through all of Snake Dance - that zone is looong. Anyway, here's a shot of my Warrior standing in front of the fearsome wurm, taken back when there were other reasons to explore that zone.
(No, that's not Mending on me, it's Holy Veil!

)
This is completely off-topic, but here's a slightly amusing picture of a ranger Tundra Giant boss found in Snake Dance, Thul Boulderbrain: