There are generally two different things that can cause this.
The first is that it won’t work at all on elite mobs.
The second is that you’re not high enough level for the game to see the mobs as trivial to you. (I think you need to be 10 levels higher, which can be an issue as you start getting the overlevel bonus at 5 levels over. It basically creates a level deadzone for those types of quests.) Every time I run into this, I submit a bug report hoping they’ll change the foam sword to 5 over.
If the character is a clothie, then picking up a 1 damage wand from a vendor can often work on such quests. (There are very few things that scale wand damage, so you’re mostly just dealing with the overlevel bonus and not stuff like your Int bonus for them.) The Whole Body Shrinka toy can be used to weaken yourself enough to not kill stuff for other classes, but it also dramatically nerfs your movement speed, so make sure you’re where you need to be when you use it.
Character at 60 or lower can turn on Chromie Time as well. You can also find someone and use party sync to scale yourself down.