Just beat the game on Legendary.
It was difficult. It was insane. It make me want to kill someone at times.
But I loved every second of it.
I am proud that Halo 4 was the first game I played on legendary. It made the emotional experience of the story so much better.
Halo 5, Bring. It. On.
Edit: Oh God Dammit, apparently the game decided "HAH, HEY BRO NOT GONNA REGISTER THAT YOU BEAT ME ON LEGENDARY" for two of the levels, so I have to do them again to get the Mk VI stuff...
DOUBLE EDIT: Make that three missions. f**k me....