Fire will instantly kill any Pure Flood, and there happen to be a good amount of Firebombs and Flamethrowers in Cortana (the first is near the Pelican you end the level at, and the second is tucked in a back hallway not too long before the engine room IIRC). You can just tap the trigger and delete any Pure Flood in front of you in an instant, and chuck Firebombs at any ranged forms that are too far away. Fire doesn't instantly kill normal Flood, but the plethora of swords through the level will keep you covered.
The real worst part of Cortana is all the fucking Gravemind/Cortana moments. You walk two feet and bam, time to walk slower than a glacier while you listen to Gravemind getting pissed and Cortana crying a lot. On the bright side, the Acrophobia skull is unaffected by the slowdown, so you can just zip right along and get through the level in minutes, useful if you're going after the Cortana terminal or the skull.