Just to add to
@Killer Angel 's points on death, in addition to the stacking of 'extra saves', there is the possibility of what I affectionally refer to as the
Hammer Time strat, in which you can't touch them.
It centers around Neferata's command ability (-1 to hit on all enemies in a bubble up to 15" from her), Bat Swarms (-1 to hit in shotting phase when within 10" of them) and the Cursed Book Artefact (-1 to hit within 3" of the bearer). This generally means that you are almost always at -1 to hit against them either at range or in combat. The next step is to surround Neferata and the Artefact bearer (Usually a winged Vampire lord for efficiency) with Hexwraiths, and for neferata to cast her unique spell on herself. This then means that not only is everything targetting this blob at -1/-2 to hit, almost every unit ignores rend. And then they get their 6+ 'extra save' and heal every turn, all the while moving at ~12" a turn. Scary stuff.
Thankfully, this is only feasible in higher points costs, and mortal wounds are definitely the way to crack this open. Go Kroak or go home basically.
In addition, as a general rule against Death-
always wipe the unit.
Even if it feels like you are using excessive resources, go the extra mile and ensure that a unit is completely destroyed, else the likes of all Flesh eaters courts units, Skellies, Zombies and a disturbing number of Tomb Kings units can just regen back up for free. If you remove the whole unit, they have to pay points to summon them back to the board and they lose their momentum.
And one last point- if you can't guarantee you can kill a big blob in the first turn,
aim for the big ones. Get rid of their Heroes, and their 'extra saves' go out the window with them, and you can mow down the chump infantry at your leisure.