Oh god I remember this mission, I got A rank first time on ace but could never get it on elite...
Basically you gotta "chain" kill everything, you see a group of enemies, kill them without...WITHOUT letting go of the trigger.
But don't just hold that button down the entire mission, breaks the combo thing, and...team mates don't help all that much, you need to concentrate on infantry, environmental stuff doesn't help unless it shoots like those turrents.
Also you are allowed to die, I had 1200 on elite flawless run, but I managed to (forgot score but from what you posted I'd of scrapped it) get A rank despite at least 6 deaths...I focussed on getting chained kills.
tldr see group, mow them down without letting go of trigger, release when dead, rinse and repeat.
Sorry for LONG delay to your question, judging from the date.