I totally want to play that achievement now. If anyone from 343 is reading this The Grunt 281 deserves full credit (/abuse from all the frustrated playthroughs ended by a BoB getting away)
Thinking about that, it's impossible to kill all enemies in Tip of the Spear
as the Falcon ride still spawns the enemies on the side the player isn't covering.
Those mysterious Banshees at the BXR mine don't always show up but it's possible they end up stuck out of view (oh and there is nothing effective at long range anyway) . That's the problem with an achievement like that : enemies stuck and hidden when it's not done on purpose. And for areas that are unloaded with enemies I don't know how an achievement should handle this.
Maybe a good % of the enemy count instead of all. Also there is more missions on other Halos (Assault on the Control Room
is a good start) .