All one has to do is read Greek mythology to realize that the Greek gods are hardly invincible. They are just a bunch of whiny stuck up beings who think they are better than everyone else.
It is clear that they are immortal. As the illiad and Odyssey refer to Zeus as the immortal Zeus. However in the Illiad Ares fights Diomedes. Now Diomedes doesn't do that well without Athena's help, but Ares doesn't outright slaughter him either. Also Diomedes is able to wound Aphrodite. In which she has to retreat from battle.
Also there is a side story about two brothers who were giants of men. In there future they were going to climb up Mount Olympus and kill the gods. They would have been able to as well, had Zeus not struck them down at an early age.
The Greek gods overthrew other immortal creatures before they could even take over the world. So being immortal in Greek Mythology wasn't really saying much.
Not to mention that Greek Mythology was full of half gods and demi gods getting killed.
Then you take a look at Marvel. Open up a wiki page and click on any name in blue. You will quickly come to realize that there universe is full of so many over powered characters it isn't even funny.
Even there non god characters are more impressive then the Greek gods. So just because Kratos can kill a god doesn't mean much in terms of being able to beat the Hulk.
It is a ridiculous and impossible battle for Kratos to ever win.
