Am I too weak or are white phantoms too strong?

I've started a new game as a pyromancer, just defeated the taurus demon in the Undead Burg. However, I was just able to defeat him using the help of a white phantom. I've tried to beat him serveral times on my own, trying to farm on the enemies every now and then. After I gave in and summoned a phantom, the fight was too easy. I went to kill the archers, the phantom positioned himself at the other end of the bridge. I went to pull the boss, who then got backstabbed to half-life immediately. I couldn't even hit the boss before the phantom took him down...

Am I too weak at this point or are white phantoms (or this one) stronger than myself?

If the Phantom was effecitvely able to 2-hit the Taurus demon, yeah. They're overlevelled for this area.

You, on the other hand, perhaps just need to take a different approach. The Taurus Demon has a fairly high resistance to fire damage, and since the Pyromancer has a fairly low "armor level" (physical damage resistance), you're actually probably better off using the axe you get instead.

The method of going off the tower to plunge-attack is a legitimate method, safe, and easily repeatable.

Some highly skilled players have managed to beat the game without any levelling at all, so there's technically no such thing as being "underlevelled"; it's just a matter of tactics.