I've done it but it's very hard. I'm a level 25 warrior, geared toward 2 hand weapons, but I had to change it to beat Undermother. First, I had to get INT 3 to get the Magical Training skill to use scrolls. Then I got Spiritual Ward III for +30 spirit resistance.
I geared up with all the spirit resistance gear I had including: Ironfoot boots (+8 spirit), Holy symbol of the Three necklace (+20), Myrosian Breastplate (+4), Ring of Unlife (+10), for a total spirit resistance of +78.
I bought 50 scrolls of Circle of Restoration that heals 50% HP, but more importantly revives your unconscious party member, which for me was level 23 Grissenda.
It took me about 15 tries to figure it out, but basically I just used the Charge IV skill, Bash III skill, and Whirlwind III skill on Undermother. The rest of the time, I was running around in circles using the Circle of Restoration scroll and Potions of Greater Healing. I let Grissenda keep hacking away at Undermother, then when she got paralyzed or died, I'd revive her with a scroll and let her use a Greater Potion. When my skills were charged I'd use them, then keep running.
Basically, if you get paralyzed even once it's all over. Even with the Ankh of Restoration, Undermother will kill you quickly if you get paralyzed. Undermother would kill Grissenda in about 4-6 hits after being revived. I used 21 Circle of Restoration scrolls and about 44 Greater Potions of Healing.
