How do you make use of destruction magic? Someone said they used archmage robes and morokei on legendary and then boasted how anyone who complained about magic being under-powered was lying.

Is there a way to play legendary difficulty without having to constantly rely on impact stun lock?

Impact. And lots of Magicka. Be sure to stay away from the actually fight. Mind [censored] them or use Conjuration.

Yes, I'm curios as well.
How effective is destruction on Legendary without stunlock?

Its fine, just make sure to get enough magicka and use pets or followers to take the damage. I don't use impact because I always keep a summon loaded up as you have to spam them a lot then just chip away at them with whatever spell you like. I usually use one of the wall spells and just set the whole area active. Poison rune works well also since its AOE damage.

Destruction without alchemy boost is like melee without smithing, not very effective. Invest in alchemy, and you can make potions that make your destruction 150 % stronger. Very useful. Also on legendary you want to use conjuration to distract the enemies from you. I use a 2 Magicka to 1 Health ration when leveling up my battle mage. With my pure mage without armor, 4 - 1, since he relies on flesh spells more. Hope I could help. OH and if you are good with wards, get the apprentice stone, an extra 100 percent magicka regen rate.