Cortana is actually very easy if you want it to be. There's invisibility hidden throughout the level; it's almost possible to get all the way to Cortana without engaging more than a handful Flood if you're quick and know where you're going. That didn't stop me from blasting through and killing every last Flood when I had seven skulls activated on solo legendary (famine was really the only one that made Cortana any more difficult than usual).
I'm not big on achievement whoring. I tend to go after them if I think they sound fun or if they have unlockables. I'm not big on any type of collection achievement, though. My roommates have huge hard-ons for achievements, though.