FYI:
zzzamorak has 57 slayer and he is lvl 3 combat so it is possible to train it but it takes ages.
Apart from using xp lamps/book of knowledge there is one more thing to do. Go to the slayer master in Burthope as he gives easiest tasks, keep changing till you get a task where the monster will be aggressive (eg if you get spiders, there are some in varrock sewers that will attack you)
Now to complete the task, keep auto-retailiate off, bring food and recoil rings. Let the spiders hit you and they will take damage back, just eat the food to heal when hp low (use low healing food, cakes and strawberries are good because they will last long).