Dark Alliance

Easiest way to tell if it is a 'Elite' is it will have a certain colour above its head. For example a goblin could have a green for poison, or blue for electric, purple for curse and so on. So if it has a colour above its head and isn't a boss, it'll be the elite you need.

Any enemy with a special element (i.e red, green, blue, etc) and a symbol above their head classifies as an elite enemy. Find and kill 300 and the achievement is yours. Not 100% sure if higher difficulties will spawn more but I went for this in low difficulties when farming my legendaries because you kill them oh so much quicker and easier.