Admittedly, these are two different things.

With edges and gear to cover dex/str loss, they'll evade enough lag to flee if you're outdamaging them and they're low. After that, they quaff. Thus nothing truly kills them except for the following:

1) Evil conjurer, with or without gaunt, mostly with, as they can avoid the fight without. Now, the gaunt has to be extremely strong or they have to be beat up, but if it works, it's death. A buff devil can work, even demon if you're lucky, but it's far from guaranteed as they may just avoid fighting you.
2) Binder/trapper/poisoner. First might make flee impossible and with prep just outdamage while cheapshotting post vuln-backstab, second can lay enough for immolation to finish (of course, preventing the elf going under water/healing at the healer is essential. Pepper dust and knowing his recall comes to mind.) Third can almost outright kill them, albeit it requires a specific set of knowledge and circumstances.
3) Polearm basher, giant or non-giant, can work just well enough for them to die. Bash and cutoff come into picture when they're at bleeding or worse, before that you're bursting. Giant imho is harder because of how fast STSF will build up. Mace/polearm comes to mind - drum the vuln, switch to polearm, bash. Shouldn't be overly hard to finish if bash lands.
4) Competent orc _if he's lucky_ - notice the lucky part, everything has to go his way, like savage feeding going off at least twice and the bash not missing.
5) Competent druid on a full moon.
6) However unlikely it is to catch them in the woods, but waylay inducing flee into snare, ambush will kill all PCs in the mud, including STSF elves, especially when it's unexpected. Finding out their prep spots and being extremely patient is key here of course. They need to not see you for this to work. You need to stay camo in a single spot for RL hours sometimes. This is not for everyone.

Some builds that can try a straight melee contest and have a slight chance of winning, but have low hopes of killing. Like shifter for example before charges build up (second fight is a bad idea.) Or a battle transmuter who gets lucky with neuro, again, only in the first battle. And of course, you'd do soften, disrupt, disrupt, and then try neuro for the finish, bailing if it fails and avoiding the fight for the login because you're getting mauled too fast in a second bout.

Overall, if they have stats covered (and three people specifically called out do) it's extremely hard to kill them. Especially Liantae. That bastard fled from me (across chars) no less than ten times at writhing/convulsing, never to be seen again until full health. Some of the above tactics were tried, but if they're paranoid about dying, it's really really hard to kill them.