Here's a good way of killing elites or hunters in legendary. get a
needler and some ammo and just unload it on an elite and watch the
show --NOTE-- You have to get in close enough for the needles to find
their mark, and the delay until they explode leaves you wide open for
their plasma rifle, so bring a backup weapon.

For the hunters, unload an entire clip at their backs so the shot doesn't
reflect off their shield and start shooting at them with another
weapon while waiting for the needles to explode (or just sneak up on
one and smack him in the back, and if you cant to that...a well placed
plasma grenade to the head should weaken it, but it will most likely
just piss it off.)