Just pace yourself, use your pet and you will do it. I beat it with a Ranger that I balanced melee skills with. I had strength and dexterity each at around around 67 and I beat it using only explosive arrows, regular arrow attack with Ursula's Bow and my creature. I released Behrlihn as well, so I had to fight him and Ygerna at the same time.
It took about 10 minutes of running around the outside of the platform and across as I evaded her rush attack which could kill me in 2-3 hits. As long as you avoid this, you should be fine. After Behrlihn and the Demon are dead, your creature really buys you some time to take Ygerna's health down.
In conclusion, a specialized class is totally not necessary, as I did it with a ranger who was evenly balanced between melee and bow. You just have to be careful, time your shots and get into a flow. That said, I too was really surprised by the difficulty spike in this last battle. Took me about 4 times to find the right pace and timing, but it is truly doable. Don't start over, just stick with it.
