In dem Video sieht man, dass bei 1:39 der Jormungar den Meleehit abbekommt, obwohl er untergetaucht ist. Was er nicht abbekommen hat ist dieser Ground Slam. Da weisen allerdings alle WotLK Quellen darauf hin, dass man nicht nur abtauchen muss, sondern eben auch hinter den Eisenkoloss gehen muss, um davon nicht mehr getroffen zu werden.
https://www.wowhead.com/wotlk/quest=13007/the-iron-colossus#comments:id=380681:reply=373205
By donjo (1,722 – 14·16) on 2008/10/09 (Patch 3.0.2)
And when he dose the ground slam you just need to run through his legs
https://www.wowhead.com/wotlk/quest=13007/the-iron-colossus#comments:id=400573:reply=435386
HappyBob on 2009/12/20 (Patch 3.3.0)
When he start to cast, i go underground and go behind Him
https://www.wowhead.com/wotlk/quest=13007/the-iron-colossus#comments:id=473631
By 253833 (1,010 – 9) on 2008/11/27 (Patch 3.0.3)
5. Whenever you see an Emote in the middle of your screen, that he starts casting something, you have 5 seconds to get away from his face. At that point, you want to submerge (the 1 key when over ground) and go behind him.
https://www.wowhead.com/wotlk/quest=13007/the-iron-colossus#comments:id=492436
By fnuffer (577 – 5) on 2008/12/06 (Patch 3.0.3)
That is because going underground provides no immunity to attacks. It only allows you to move. So take the hits, and when he starts to cast slam, submerge and move behind him.
Von 
Von