If you read the Wisdom Drain(SU) ability of a Lamia Matriarch it says "A lamia matriarch drains 1d4 points of Wisdom each time she hits with her melee touch attack. The first time each round that she strikes a foe with a melee weapon, she also drains 1 point of Wisdom."
That seems to indicate the intention is for the matriarch either perform a touch attack, or get 1 wisdom drain with their first successful itterative attack. If you decided to make a monk version of a Lamia I'd think it would be best to limit the wisdom drain to 1 point per strike. Though there is also the Lamia Hungerer you could consider, which does 2 points of wisdom damage per attack.
Most characters don't have a lot of wisdom. Typically 10 points is enough to incapacitate a player. Having a monster that does 3d4 wisdom damage a round is insane. The Hungerer does potentially 6 points a round and when I was running RoTRL it did take out characters in 2 rounds. Don't go all gung ho with stat damage. As a GM putting it in as a threat is good, but putting in too much of it is unfair.
Yes this is actually to get a fair ruling from our DM in ROTRL, he ruled it as : 1 point of damage every weapon hit, made her a swashbuckler, used opportune parry and riposte to slap back our barb once a round with 2d4 wisdom damage. Then also made touch attacks every round for that. Our barb went down every round or so and we have to feed her potions of lesser resto to get her back in the fight.
But aside from my own situation, I'd really enjoy knowing just to know.