Even if the SA damage won't scale with the new helpless change those epic mobs will melt under your attacks.
As soon as you get your radiance weapon soloing anything with sneakable enemies won't be any trouble.
Before that you can get along with good dual deception weapons (they work on rednamed too).
Or you let your hireling grab aggro, assassinate everything one by one and switch to deception again for the boss.
If you go assassin you get a free double (triple->doublestrike) instakill with a DC37ish fort save at lvl 20 every 15 seconds.
Even in elite amrath quests it lands quite often on orthons/devils, and it works pretty reliable on the casters in there.
Add inherent vorpals at lvl 18 to that and you won't look back to fighters or rangers.
Beware: May cause heavy addiction to sneak attack damage.
Haven't run my ranger for over a year now for his lack of tasty SA dice