Hmmm, how many Undead actually have Lifesense? It is not a universal undead trait or even a common ability for Undead to have. (As in the only one I'm aware of is the Wraith/Dread Wraith off the top of my head). Of course if your GM has homeruled that Undead in general have Lifesense (as it makes a certain sort of sense) then you do have a problem.
You use the word sniper ... do you mean attack from distance -> especially greater distances? Lifesense by default only has a 60ft range. If you attack from 100ft (i.e. >60ft) they have no unusual advantage in spotting/perceiving you.
Maybe create the equivalent of a Ghille suit that masks your aura?? Along these lines what resources are you allowed? I don't have access to my copy of Libris Mortis but I'm going to guess there are items in there that would be useful.
Ethereal creatures can not be detected by Lifesense. And while you can not attack them while Ethereal you could wait and watch though it might be a bit distorted. Then "phase-in" ala a Phase Spider to attack (The problem is doing so is a relatively high level tactic)
Maybe just hide in your own Rope Trick. You can see out (though not attack out generally) and elect to move out when it is to your advantage (or at least not disadvantage) to do so.
Antimagic or similar will block Lifesense (but I suspect also nuke your own abilities and therefore not desirable).
Or as implied by Hawktitan become/polymorph into something 'not living'. Very similar to the camo/ghillie suit idea.
