I can only speak for Bronze rankings. For this type of mission depending on which wave it is you will get stronger monsters that spawn plus the trash monsters. After the stronger monsters are dead, the trash monsters will never stop spawning, they will also continue to spawn in the same area until your team moves to a different area or completes one of the 4 disarmings.
I have done this a few times with reapers as an enemy, I forgot which wave, but there was 2 banshees and 2 brutes and 3 or 4 ravagers. After all those enemies were dead the only monsters that continued to spawn were cannibals and maruder and rarely a few husks would come running.
I forgot the numbers now and dont have it in front of me at work, but I think 8 enemies can spawn at once and only a maxium of 2 spawn points at a time. So from what I have seen, no facts behind this. So if you know where current spawn point is you can run the backways around the map as they head to your last location (as long as you dont shoot) and than can get the next objective and never have to shoot at the trash, and until you kill the trash more wont spawn.
But I have stood in the same spot for 4 minutes with a sniper rifle and let them run down the hallway and killed 20-30 monsters in the 4 minutes because spawn point didnt change because I did not move. 4 minutes ended, my mission ended, but got like 20k score.
Pull - Grabs the shield out of an enemy's hands, leaving them exposed.
Stasis Bubble - Stasis Bubble (level 6 stasis upgrade) will cause an enemy to drop their shield. Sometimes they seem to block the initial hit, but as soon as they move at all, they get caught in the bubble trap and the shield falls.
Armor Piercing Ammo / Piercing Mods / Widow, Black Widow, and Javelin Sniper Rifles - Ammo, mods, and weapons that provide an armor-piercing capability allow you to fire THROUGH a riot shield (and other light cover). The M-98 Widow, Black Widow, and Javelin sniper rifles have armor-piercing rounds by default (no ammo or mods necessary). In multiplayer, armor-piercing rounds can be purchased at the store and equipped on the "Equipment" screen.
Well-Aimed Headshots - The Cerberus Guardian's riot shield has a view slit right at head level, and you can shoot through the slit and score headshots if you've got good aim and an accurate weapon. The "Mail Slot" achievement/trophy is awarded for killing 10 guardians in this fashion.
Singularity - If you aim above or to the side of an enemy holding a shield, sometimes you can get a singularity to "curve" and hit the enemy from the side or behind, avoiding their shield. The enemy will drop their shield if they are caught in the singularity. It's a little tricky, and doesn't work in all situations, but with some practice it can be effective.
Forceful Attacks - Many special powers will cause an enemy with a shield to stumble and lower the shield temporarily, giving you a window of opportunity to shoot them while their shield isn't blocking them. Includes, but is not limited to: Throw, Shockwave, Charge, Nova, Concussive Shot, etc...
Flank / Combat Drone - If you and a teammate can flank a enemy with a shield, one of you should have a clear shot - The shield only faces one direction. In the absence of a teammate, a Combat Drone can be deployed behind or to the side of the enemy to attract their attention.
Best Answer
Yes, enemies have scaled with level since Mass Effect 1 and Mass Effect 3 appears to be no exception from my experience.
Since an imported level 30 character experiences a normal difficulty in the start of the game rather than being totally overpowered I think it's quite apparent that difficulty does scale with level. If enemies didn't scale with level, imported characters would be totally broken so it's easy to see why they kept the scaling difficulty.