The person is complaining that the stonefall from the slasher kills the player's character in one hit when they attempt to solo.
Technically, the stonefall is actually a succession of attacks while the characters are frozen.
I've rarely known it to let someone LIVE if they get hit with it. Usually the cumulative attacks are enough to kill anyone through max resists and an active bandage going off inside it (Started before the attack), unless they are at the fringe of the attack. This seems to indicate that the attacks have a range modifier.
And, yes, chain lightning and meteor swarm have damage that scales to the number of targets (IIRC, it takes the base damage, multiplies it by 3, then divides it by the number of valid targets (or 2, if there's only one valid target)). But, I don't think that's what's happening at Slasher.
The reason why someone typically gets hit by the worst of the attack is that they are in melee range, when it is initiated - and if there's no other nearby targets for the slasher to switch to, it just sits there keeping the worst over the character. Typically, there will be a hell hound or another person fighting it to distract it - if you lead off the hell hounds and are doing it solo, you are bound to catch the attack a lot worse.