Let's say you have to bind a certain amount of your hp to a monster, for example say you summon a skeleton, for every 10 damage it recieves you'll lose a hitpoint, also, keep in mind you can't outfit them with armour and weapons, so it'd still be more worthwhile to use combat yourself and have a summon attack alongside you, but with you soaking up the damage. The highest level summoned monster could be a greater demon? I don't think they'd go so far as to let you summon boss monsters anywhere in runescape..that binding idea is good maybe.