At first I played up to about 20 bloodlines and every time the god king defeated me. However IF you choose to join him instead of picking up your sword, all he says is "Together we will vanquish the old corrupt more powerful beings than the deathless.." and then you can choose to fight him again..so joining him is pointless. You know how your enemies become stronger, faster, and harder to defeat? Well the god king is always the same: just hard to kill.
So yes I believe it's possible to defeat him on the first bloodline. Say you bought the Infinity Blade, The Patriot (shield), Oruku (helmet), Helio Amrmor,(all the best supplies but ones to REALLY save up for) and the Ring of Ice and Fire, and you were on the first bloodline, you can defeat him by merely avoiding his attacks. However it seems the makers of the game don't let you use your shield sometimes while you fight him, or make the controls slower so you get damaged.
The question is: What happens IF and after we defeat the deathless god king?