Harder than Ornstein and Smough.
It took a fair bit of effort to reach this boss room, but that's to be expected as the last lord soul is within reach. I found the sickly Prince Lothric waiting on his throne, but he wasn't going to do any fighting. He left that up to his much larger brother, Lorian who is a complete, teleporting, melee asskicker. Since this fight was killing me more than Ornstein and Smough I tried to summon help from players Turms, Gwyndolin and sunbro Cheshire Charr (go the GW2 reference!) who unfortunately could not enter the boss chamber. Thanks for trying guys!
Since it looked like I'd need to do this solo, I switched out my axe for Tsorig's Fume Greatsword - upgrading it fully for maximum reach and damage. It took a few attempt but finally, I slew Lorian! And then... Lothric teleports onto his back and ressurects him for phase 2... which now includes Lothric's many heatseeking, magic attacks. OMGWTFBBQ!?!
After quitting for a day, I decided to try enlist help again - this time from sunbros Yen Sid the mage and Tartarus the knight. This time they BOTH could enter, and we absolutely slammed the princely brothers into the dirt! Finally, all the lord souls were collected and back at Firelink Shrine the Firekeeper performed a ritual to take their flames (killing poor Ludeth) and pour them onto me - so that I may finally face the old gods of Lordran.
Best solution is again to bring numbers!