Sorry I am a bit late, but I will post my build and how I killed the unweakened Undermother relatively easily, without spending tons of cash on potions. PS: I am a pure ranged rogue, so it didn't take massive amounts of skill. My build is unfinished as of yet. I am currently Lv 18, was 17 at the time. This is my first and only character, so pls be gentle if what I am saying is painfully obvious.
Companion = Grissenda, standard tank SnS, but in this fight, she was unimportant. She was incapacitated the whole fight.
My traits skills and equipment are slightly more developed now than they were when I killed the Undermother, but I am squishy enough to be killed by monsters a lot lower Lv than me if I get lazy or make a silly mistake. A lot of previous bosses were harder than the Undermother for this character.
This is what I used to kill her at the time:
Equipment:
Tiara of the Heavens
Primal Ice Chain Shirt
Gloves of the Explorer
Bloodied Scarf
Poisonous Longbow & Jester's Bow
Ring of Unlife (X2)
Girdle of the Rogue
Ironfoot Boots
Lionskin Leggings
Ironwill Mantle
Skills:
Evasion III
Trap Master III
Archery III
Extra Recovery II
Explosive Traps III
Precision Shots II
Traits:
8 Agility +2 from Girdle of the Rogue
5 Awareness
1 Intellect +1 from Ring(s) of Unlife/Tiara of the Heavens (trait bonuses of the same type don't stack)
The gear and traits aren't really that important, as long as you have Archery III Trap Master I at least, and Bloodied scarf and Poisonous Longbow. Make sure the area is completely clear of Mirmeks beforehand.
The strategy is to trap the Undermother (move speed reduction) and attack her with the Jester's Bow until a stun proc, the high ATK speed = more stuns with archery III, and you can attack while kiting even without the move speed reduction.
When she is stunned I would switch to the Poisonous Longbow (via Quick slots) to do more damage, then swap back to the Jester's Bow after 3 or 4 hits. The Undermother is not a fast enemy (like Great Manticore or Vorator), so you really shouldn't get hit. If I felt I was at risk, I would use Evasion. Save it for close calls.
Just in case, I had on the Bloodied scarf (stun protection and half paralysis duration) and gear which amounts to 53 Spirit resistance, so in case I did get hit, I would have approximately 25% to halve the duration (which would mean 1/4 in theory, but I didn't get hit), and 25% to negate it completely.
The heals were a pain, but I don't think the Undermother has unlimited mana, because after 4 or so heal attempts (2 of which were interrupted by a stun) she stopped healing herself. It was a long fight, but not hard. Expect sore hands afterwards.
Love the game, Hope this helps somebody.
Here is my character now, as proof (I play on the iPad).