Nice ideas there Hardhead, and one of the few (the only) I've seen that might actually do something. I wouldn't say it'd be the desired effect, just that it would have an effect. The Last Word we know can slay deities instantly, however we don't know the upper boundary of its power, or other things about it.
For instance, does the target deity have to be manifested physically to get nailed with it? The Lady doesn't have to appear to maze or flay anything for certain, so would you even have the chance to use it? Would The Lady be hurt by it? Would She laugh at your use of it? Would She do nothing and then reply in the same language? All open questions that don't have concrete answers, but thats half the fun of working with them.
And on a second point, it's been interesting to see 'The Serpent' change from its earliest mention in the Vecna modules where it was never in reference to an actual being, it was just Vecna's way of personifying magic as more than just a benign force.
Eventually other authors altered that till The Serpent became (in Die Vecna Die) an actual being of sorts that used Vecna for its own purposes, etc.
And The Lady of Pain versus the Tarasque. After all, noone knows just what, if anything, was ever kept locked away behind the rusted iron bars of The Gatehouse in Sigil. My own interpretation was that it may have been The Mother of Serpents kept locked away there at some point in the dim forgotten past, but of course there's nothing to prove that.
But that fight would go something like this, if She allowed the beast to enter Sigil in the first place.
Round 1: The Lady goes first and Her shadow transfixes the Tarasque. The Tarasque shudders as it erupts into a boiling cloud of blood and screams.
Round 2-20: More blood and more tortured screaming, gradually fading away
Round 21: The shadow vanishes, the blood settles to the street like a fine rain and reveals nothing but a large smear where the Tarasque once stood
Actually come to think of it, thats how alot of encounters with The Lady would end...