So I've heard Alduin, during one of his consumptions of the Kalpa, he ate a begging Mehrunes Dagon and that was what turned him into the Daedric Lord we see today.

But I don't understand how an eater of worlds can be defeated by a one eyed warrior, a battle-hardened woman (she seems to be the toughest out of the three and Alduin kills her first), and an old man who uses magic. Even then the only reason they won is because of the Elder Scroll.

Then in Sovngarde, these same warriors and the supposed last Dragonborn defeats Alduin in a toe to toe battle.

So just how powerful is Alduin in relation to the Daedric Lords? Because he seems really....weak. I know its in-game, but still, he's the "World Eater" and he loses to four warriors gifted with the Thuu'um.

Alduin gains strength through...well, devouring things of course. I figure we managed to off him fairly early in the game so to speak, so he really didn't have any time to amass a lot of power. The Tongues defeated him and tossed him through time (Which, for an aspect of the Time Dragon must have been extremely unpleasant.) Also, keep in mind that Dragonrend was specifically created for bringing down Alduin. Without it, hes practically nigh invulnerable.