An easy way to deal with them, will be to cast Doom to lower the slime's saving throws and then using a Wand of Polymorphing to turn it into a squirrel. This bypass the need for them to die by "fire attacks". Although the polymorphed squirrel's death animation shows it splitting into 2 slimes, the party will almost immediately be teleported into the next part of the Durlag's Tower challenge.
- It should be noted that they have to die from a fire-based attack, either magical or non-magical (by arrows), or they will split into two more slimes. If the split bugs, i.e. not dying to fire, or if one is using the "cheat keys" namely CTRL-Y, you will be bugged in Durlag's Tower and cannot advance without the use of a script. More info can be found on that particular bug here.