Really the issue is that we don't really know what Iridikron is planning or even what his motives are. Razsageth and possibly Vyranoth may be idealogues; they truly want to remove the influence of the Titans. But Iridikron is likely to have a much more base desire from the way he has been described. If he is after power, maybe he is after the World Soul (everyone else is). If he is also affected by the Void, someone needs to replace the writer team already.
- - - Updated - - -
Honestly I think a good model for how a fight against something like Deathwing could have worked is Sinestra and later Helya. Simply remove the mobility from the boss itself allows it to be more massive. If you want a giant creature to attack the players, focus on area attacks instead of attacking the tanks (Helya's tentacle attacks).