Oh you are right, I got that wrong. It should be a mortal wound IMO. It is deadly poison after all.
To be honest, i'd say do the following:
1) 6+ hit causes bonus mortal wounds instead of bonus damage, or at least removes the need for a wound roll....
2) Some form of additional protection from ranged attacks, regardless of scenery. They're chameleons they should be difficult to spot and most ranged attacks aimed at them should end up missing. This includes spells & abilities and what not. For example, do the following, when attacked from more than 3"away, roll a D6 on a 5+ the enemy just misses. It'd encourage an enemy to activly chase em instead of just firing 1 basic spell in and wiping out half of the unit completly crippling it for the rest of the game... Which for an "elite" unit is kind of silly... Possibly in addition to the current save bonus or as an alternative. Not sure how powerfull this'd ultimatly be.
3) 2 wounds. I seriously hate that our "elite" options of our normal infantry only have 1 wound... it just rubs me the wrong way.
4) They can reappear immeadiatly in the next movement phase, so they don't spend half their time off the board. Possibly with some trade-off, for example for each hero phase they hide they gain a bonus attack when appearing? Could help with them actually being a functional assasin unit.
5) Something to up their sustained damage, and something to up their burst damage., preferably in the form of a trade-off. For example a mechanic along the lines of +1 to hit rolls if they didn't move in the previous turn alongside what I mentioned in point 4. This way they'd be a bit more bursty when trying to assasinate a priority target, but more sustainable when they sit in a nice spot and just fire away into enemy lines.
Do that and they actually become more than merely "annoying" like they mostly are now...