First order of business when dealing with a hostile armed with a Needler is finding cover; second order of business is killing the mike foxtrot with the Needler.
Foxtrot is an expression used and utilized by UNSC Marines in the active field of combat, and is also the second part of the level Sierra 117 in Halo 3. A Charlie Foxtrot (or a 'cluster fuck') was originally used in the United States Army in the two World Wars, and is slang, that basically means when a group of soldiers 'screw up', such as if they are unprepared or disorganized on the battle field but it can also mean that command has screwed up or is doing things unfitting for Marines or Soldiers, such as making them fill out request forms for equipment they should already have, etc. Mike Foxtrot is another term for "mother fucker," whilst simply "foxtrot" means simply "fuck."
In the Halo Universe, it is a phrase used by the UNSC (the expression had presumably carried on throughout the American Army's history) and it means the same as it did in the past. It is often said when Pelican dropships are about to be shot down. It is also commonly used as an insult towards Covenant forces (when a Covenant member is killed by a Marine, the Marine will often yell out "Foxtrot!" and repeatedly shoot the dead body). Foxtrot is also the F in the Phonetic Alphabet.