You could try magical - it may turn out to be effective - but generally it is expensive (200GP per cast with OSRS gold the Polypore is still pricey, comparatively ).

Secondly, utilize higher-defence armour till you become proficient at Prayer switching at TDS, then switch to Void. How would you're good at prayer changing? If they are attacking with melee and then they roar then you've got to guess involving range or mage. If your guess is right then it's fine but when it's wrong then you're likely going to require damage before you switch again...

Correct me if I am wrong... it's just what I read on manuals I never moved to there or even done the quest yet. So it is a blueprint and they do not just switch their strikes randomly, right? Nah it's random, it is just that they don't really hit with melee at all, therefore it's better to pray agaisnt magic or variety. If they are about to begin ranging, when they roar, a tiny bone shard item will appear, and move upward, though it can be somewhat tricky to spot occasionally. Keep an eye out when the demon's simply been meleeing.

At slayer tower you'd kill them quicker with cheap RuneScape gold melee so if that is what you want go with that path. Armor should be your best magic defense (Attack is melee but it's defense is based off magical so d'hide and the like) or in case you really feel like praying (Bit of a waste unless you are fighting the mutated ones with a cannon) then greatest prayer protection and pray protect melee. For weapons go with your best slash weapon.