There is a lot of misinformation being spread around by the veterans on the official discord about the role enemies play in the missions, so lets talk about a few of them and there purpose, and hopefully this will be helpful to some of you.
Horde Type
The point of these enemies is to distract you, and slow you down. If you are switching to melee, to kill the horde, swinging away at it for a few minutes, you are playing into the developers hand, and falling for there trick. In essence the horde is a distractor.
Without the horde, elites/specialists would be useless, the horde is what sets you up for them to take advantage. Hordes are divided into 4 types, peasants in robes, infested peasants in robes, angry shirtless dudes, angry armored dudes. They all serve the same purpose, some are just better at dealing damage, others are better at surviving longer and being more of a nuisance.
Damage Dealers
Ogrynns, maulers, ragers, shotgunners fall into this category. They take advantage of you being distracted by the horde, and it reducing your field of vision, in order to get close to you and wreck your face. This is why you do not waste time meleeing the horde.
Exploiters
Trappers/dog/bursters/flamers are meant to take advantage of you being held in place by the horde and specialists. Engaging the horde with melee, making the event last longer, puts you at further risk of falling victim to any of these.
Sniper/gunners
Contrary to what most of you might think, the sniper/gunner job is not to kill you, but to make sure all of you cower behind a corner, and keep you from advancing. His job is to stop you from moving. The majority of players I encounter in matches, play right into the snipers/gunners trap, they either run back and hide, and play peek a boo, or they freeze in place and get wrecked by something else.
SOOOO
So yeah, its up to you guys to come up with your way of dealing with these threats. Now that you know what there purpose is, it is a lot easier to deal with them in the appropiate manner, and for you to increase your chances of success.