Health early on has great value, that's why Runeforged Hammer is my top first item. Then Prophetic Cloath in order to gain some tankiness and be able to stack it. Then comes Bragi's Harp because it suits very well for a God like Ymir, with STR and INT + AS + a nice AA passive. We go then for a beserker shield in order to improve both tankiness but more importantly our AS. As a 5th item, Blinking Abyss for both agressive and defensive purposes (depends on the situation you're facing). And we finish with a Tekko-Kagi to improve your kill potential. The only issue with this build is the lack of pen, so if you face very tanky gods (2 tanks + 1 bruiser) you can consider then some pen item.
If their team is very tanky, and especially if their tanks/bruisers gives you a lot of difficulties, consider to buy Titan's Bane in order to be able to delete them before they delete you.
