Godzilla was obviously out of either Mutos' league one-on-one. But as a duo they were a powerful team, and even had coordinated attacks. They weren't fighting just for survival, they were methodically timing their attacks.
As for Godzilla's dying.....
He died in the 1954 original--the rest of the Showa series (1955-1975) doesn't really outright say whether Godzilla is the same creature as the first film or not. "Gigantis" in 1955 was implied to be another creature from the original film, but there are so many inconsistencies in that series who knows.
He was "killed" in Godzilla vs. MechaGodzilla 2 but was revived by the dying Rodan's essence. It's a dumb part of the series anyway. They added a second brain in Godzilla's body to attack so that he can be paralyzed and killed. But no other film ever acknowledges this. Same with the phantom "armpit" weakness they made up for Godzilla vs. Space Godzilla.
Godzilla obviously died in 1995 in "Destroyah", but his death revived and transformed Junior into a new Godzilla.
In Godzilla 2000, they imply that that Godzilla is probably the one from 1954 and he completely regenerated from atoms. He's like the Wolverine comparison.
GMK, some might say, was killed at the end of the film. But a beating heart tells me otherwise, especially since Godzilla's whole backstory in that film is an unstoppable spiritual nature. If the souls that congregate in Godzilla can't die, neither can Godzilla. He's a dark god. He'd probably come back stronger. But I think this Godzilla is outright said to be the same one that attacked Tokyo in 1954.
And as nam said, Godzilla in the SOS series has to be a different animal since he's fighting the bones of the original.
All the other times, Godzilla has been trapped or dumped in the ocean not returning for the rest of that film, such as Gigantis, King Kong vs Godzilla, Mothra vs. Godzilla, Monster Zero, 1984, Godzilla vs Mothra, Godzilla X Megaguirus, Tokyo SOS. He's alive, but that's how Toho temporarily feels is most convenient to end Godzilla's reign.
