The other thing, I suppose is, that it's true that Lavos has planet destroying power... but at a far distance. For example, the US may have a surfeit of nuclear weapons, but wouldn't dream (except for maybe the gravest of counsels of despair) to launch them if their own soil were under attack - it would injure them too greatly. Likewise, considering how near they stand to Lavos, it may just be he cannot use his most powerful spells and sorcery on them due to their proximity. As such, Crono would not have to withstand such high forces as might otherwise be attested to him. Taking it down to plot, it may just be a case of David and Goliath, rather than, well, Goliath against Goliath. Crono didn't win because of superior might, but by using tactics that Lavos could not counter well. Okay, David and Goliath wasn't meant to be about that, bad example. Say, rather, the second Death Star from Star Wars. It had the power to destroy planets. Yet Wedge and Lando, flying an X-wing and a heavily modified correllian transport, destroyed it - a station 120 km across. Look at Crono and Lavos the same way. Taken at sheer power, Lavos is a hundred times their greater. But what Crono did was engage him at point blank range - still a peril (for which he needed his great power that he gained through the journey), but annulled the greatest portion of Lavos' strength. Then, having blasted a hole into it, proceeds to combat its more vulnerable self under its mighty battle-armour. Just like the Death Star, which stood immortal and unstoppable for a time, but once breached was destroyed by a proton torpedo and a concussion missile. 120km of space station destroyed, because it was in-apt to combat the tactics it was presented with. The same is true for Lavos. Crono's victory wasn't all might - it was cunning, and knowing how to fight. (Now, Crono, I'd figure, is probably equal to or greater than the true Lavos inside... but from the way I figured through this, is no match to the planet destroying bio-armour he is encased in.)
Now, as such, it means that Crono isn't all THAT powerful. There is no neccessity saying he can't be killed. Yes, he may be a great hero, but he is still human. Grey, you brought up the example of Beowulf. Yes, he was very strong, and killed Grendel and his mother with his bare hands... but still he perished to the dragon! And Sigurd, the bane of the dragon Fafnir, who became nearly invincible by bathing in the dragon-blood, was killed by treachery. Tolkien's Turin, a mirror for Sigurd, who performed similar feats in the killing of a great dragon, and was amongst the best of mortal warriors (and, indeed, was fated to kill the Dark Lord Melkor, Sauron's old master, at the Last Battle), died - in grief and horror, casting himself upon his own sword. Herakles, mightiest of the Greek heroes, did not meet any natural or peaceful end. The hero that defeated the great hydra, who went to the ends of the earth to fetch the apples of the Hysperides, and for a time bore the weight of the world in place of Atlas, and later took Kerberos from Hades itself, was killed - by a dead foe, no less. The Centaur Nessus, in lies to Herakles' wife, convinced her that his blood was a love charm - but when she attempted to use it, sending her beloved husband a cloak dipped in it, the hydra venom that had been upon the arrow when Herakles killed Nessus, burned into the hero's skin, and painfully killed him. Subterfuge, again, is the bane of a hero no one - not the giants themselves - could stand against. Great Aias, second only to Akhilleus at Troy, who was like a wall in the defence of his fellows, died by his own hand in remorse. There are manifold heroes that can be mentioned, many having superhuman qualities, who nonetheless succumed to death (though apotheosis on occassion follows that.) I do not see how Crono need be any different. Indeed, like Herakles, it could even have been a minor foe, one he could easily kill, that was his downfall. No great earth-giant, no boar or bull - a ferryman centaur.
