It's fairly obvious that Luca's strength comes from himself and his own twisted mind. Seriously, that's the main reason he's so damn scary! Obviously, his training and armor gave him an edge, as did his sheer determination, but I can't completely count the Beast Rune out of his list of strengths.
I'm not saying that the rune actually gave him power the way most runes do. Rather, I think the Beast Rune provided Luca with a sense of invinciablity. After the events at Muse, it would be hard for anyone not to see the rune as a nigh unstoppable force. Luca probably thought that as long as he had that feral force to fall back on, he was as good as immortal on the battlefield.
Plus, we can't ignore the effect of the whole mind-over-matter aspect here, either. Luca did not see himself as a great general or a powerful prince; based on his last words, he likely saw himself the same way other people did - as a bloodthirsty demonic tank of a man. Luca seemed to like this persona, so much so as he wished to be remembered that way even once he realized he was moments from death. Luca feverishly believed himself to be unstoppable, and to a certain degree, he was.
Finally, I think the time frame of the fight plays a factor here. Luca's last stand seems long only because of the turn-based battle system. Hell, it took me an hour to fight him my first try and I lost! The turn system is a gameplay element, and if you ignore that, it seems more likely that each clash with a seperate group lasted only a few moments or less, just long enough for Luca to fend them off and discourage or wound them enough to simply walk away. All in all, Luca's last stand probably only lasted 10 minutes at most, which is no less impressive since he was surely in incredible pain the whole time.
Of course, I like to think that Luca may still be alive, and will drag himself into South Window some day soon and start going nuts again, but that's just me