Slimes do not use the slot.weapon for attacks

If you spawn a slime using this command:

/summon Slime ~ ~ ~ {Size:1,Equipment:[{id:276,Count:1,Damage:0,tag:{ench:[{id:20,lvl:1}]}},{},{},{},{}]}

It will summon a slime holding a diamond sword enchanted with fire aspect 1, however, the slime does not use it.

This will work with most other mobs, for example, a Silverfish:

/summon Silverfish ~ ~ ~ {Equipment:[{id:276,Count:1,Damage:0,tag:{ench:[{id:20,lvl:1}]}},{},{},{},{}]}

This command will summon a Silverfish holding a diamond sword, and he WILL use it.