from lvl 68 magic you unlock lvl 5 enchant. this takes 1 cosmic, 15 water and 15 earth runes. you can negate earth and water runes with mud battlestaff. In Magic training arena is activity called enchanting chamber. You gain 5 enchanting points per lvl 5 enchant. in magic training arena shop 1 cosmic costs 5 enchanting points. if you drop your enchanted orbs at center you get for per 20 orbs 3 blood, death, or cosmic runes.

you get 6 points with lvl 6 enchant and 7 points with lvl 7 enchant(but since there is no staff that you can use to negate non cosmic runes with lvl 7 enchant so we can skip that). you get double the points when you enchant 10th object (not dragonstone since it doesn't count towards that)

so you could essentially go in with 10 cosmic runes,mud battlestaff,lava battlestaff at 68 magic and leave magic training arena with bunch of cosmic,blood and death runes and 99 magic.