A lesser demon has 5,000 life points.
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Taunt the demon at the top of the Wizards' Tower(By choosing the option from the cage)
3, there are safe spots there...
I would say you should be level 50+. You could be lower but with better food.
XP is not calculate when you kill a monster, but every time you hit it. You get XP for every damage point you inflict. In the case of magic spells, you also get a basic magic experience, just for using the spell. I am not sure about ranged.
A quest walkthrough for Demon Slayer can be found here: www.runescape.wikia.com/wiki/Demon Slayer