The higher the difficulty, the higher incoming damage you receive, this is why a lot of ppl go for Heavy Damage , Slow Reload option aka Snipers & Heavy Pistols. Or powers.
There is no suppressing fire - enemy only panics when they are set on fire or staggered by explosions / skill use. Strangely, warp / ranged biotic throws forces the enemy to relocate or jump away.
The above contributes to certain power-based class being more effective than the rest. They don't need gears to be effective, unload plenty of damage through biotics, and employ Heavy Pistol/Snipers to finish the job.
That is why Asari Adepts are generally effective vs most enemies and human adepts are the worst.
Lesser used skills that might help:
Prox. Mines - This is a mixed bag. It's useful. But the way it deals damage is tricky to implement. At times if you stick it on an enemy, it explodes instantly for good dmg (mini grenade) and stagger. At times you miss, and it does absolutely nothing for the next few seconds till the sensors are up, usually by that point, the enemies have moved away/died. My advice is not to aim at enemies, instead, aim for the ground. Take Rank 5 Evolve - add 20% damage on affected enemies. You should plant mines during round breaks - when enemy comes close, fire another to detonate it.
Turrets & Engineers Pets - You can use them as fire support or force the enemy big guns to turn around. Very noticeable when you set those behind Atlas / Geth Primes. The elites would turn around, exposing their back. Also, Cryo ammo for them might be trick to double Incinerate Dmg and slow down enemies.
Powers - If you're comparing damage against basic troops, there won't be much of a difference whether you use a gun or power. But if you're up against hard targets like Ravager, Brutes, Primes, Hunters, etc guns alone won't be enough. Don't think any class operate without using powers. Even Soldiers rely on Adrenaline to compete. Infiltrators stealth to gain 90% dmg bonus on next attack etc.