I am really enjoying this game so far. I love the fact that there is almost no strategic base-building and more tactical sneaking. My only complaints are the buggy controls and the inability to sell items (e.g. armour) to the shop in the Ark (how does she get supplies?).
On my first playthrough, I have just finished rescuing Magnus, and since I have nowhere else to go, really, have abandoned Izza and Fala (which seems too difficult for little reward, and appears to be optional) and have gone to the Iron Serpent. I have not been looking at spoilers, but it seems this is the path I must take to rescue Hammond.
I am confused, however, since I am level 20 and all the enemies are level 25, and they are really hard here. I have nowhere else to go to level up. (The only other area I can reach is red-skulled 35.) Is this normal? Am I just supposed to push through and expect the fights to be significantly more difficult?