If you have STUN IMMUNITY, than the worst part of the Undermother is addressed (her stun attack) but she does have a lot of hit points and does some serious damage so bringing a lot of top tier healing potions will help.
With Restoration scrolls (they bring Companions back from the dead), I'd suggest using Grissenda as she has better defense and attack. However, since you don't have Magical Training, it might be better to use Hirge with a healing build. Epic Omelettes plus Hirge is a power combo (Epic Omelettes restore half of your HP/MP, but since the last update, can they be used in a fight?, I forgetting now).
Don't forget your Companion can use a potion or scroll every 3 seconds and so can you so every 3 seconds, you should be downing 2 potions. Health is good, obvious but defense and attack potions help too.
You can bring a hired companion and a summoned creature into the fight, they might not last too long but it can help spread out the early damage.
As a 1-handed warrior, you might want to go head to head with the Undermother, you'll take most the damage but that will allow your companion to last longer. You can always pop out for a second to heal if needed but don't let your Companion die.