I've played through two bloodlines so far in Infinity Blade. Both times I made it to the boss, and both times he beat me. I imagine it's possible to beat him after leveling up over the course of many tries, but is it possible to do it on the first bloodline?
I beat him on first blood line by going the way with most enemies (fighting as many as I could), and spending any skill points I got into attack, and then buying the best sword I could before the fight (for me it was the firebrand). On the first blood line, the god king is at a reasonable speed and parrying his attacks is very straightforward, and not subject to any slow controls like the dodge buttons. If you fail once, you can use your shield and dodge for the rest of the combo.
Once you stop his attack, you should perform a 3 hit combo (left right left) if you dodged, a 4 (left left right right) or 5 hit combo (up down left right up) if you parried. Use your magic in case of trouble or to finish him off (it will take 100 points out of him). Thanks to parrying you will get multiple stab opportunities that will be draining him pretty quickly too.
Once thing to remember about parrying: it is OK to parry the very last swipe of a GK combo, you'll still have a longer time to hit back at him, and sometimes get the stab opportunities. Also, there are only two orientations for parrying: horizontal and vertical, and the direction (left to right or right to left for example) doesn't matter! So there are only two choices, which makes it much easier once you have a feel for that.
If you use parrying the fight is very short (maybe 2 or three minutes) and quite easy (easier than some of the critters on the way to the GK, because they're sooooo slow that parrying them requires much better timing than the GK...). In the end you might take a hit or two, but shouldn't feel the need to heal yourself. Fire swords are good only if the person doesn't have a shield that blocks fire objects.
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.
No, you can't beat him in the first bloodline... because that's the intro. :D
On the second one, yes, you can do it by restarting from a higher bloodline after leveling up. I actually beat him at level 15, using the heal and shield ring (forgot what that was).
Level up to 15 and buy the best equipment possible (excluding armor), then choose to "Restart from Bloodline 1". You will still be facing the Level 50 God King, and so you can then beat him in Bloodline 1.
It's actually not that hard to beat him on the first bloodline. But that's probably because while I was playing, I got some weird glitch and suddenly had $10000000. I bought all the best equipment, mastered it all, (that got me to about level 30) and beat the God King and he only took about 200 health from me. So, it's actually not impossible to beat him on the first bloodline, you just have to be really good, or really lucky and get a glitch :D
Just start early on and keep playing, and then a few bloodlines later say do the bloodline one and he will be easy.
I did not do this every time, but I beat him seven times. You could also go down the drain or below the elevator to the God King and have the Infinity Blade, put it in the stone and amazing people are behind the doors and beating them opens the door to the creator of god kings.