Quarians have spent centuries at perpetual salvage and ship repair--the survival of their species depends on this technical expertise.
Quarian engineers are elite support specialists with a talent for setting up ambushes and providing cover fire through the use of turrets. Their technical wizardry makes them crucial contributors on the battlefield.
Quarian Engineers are a little more focused on dealing with armored and unprotected foes. They are less effective against barriers and shields, but this can be ameliorated by using a weapon effective against shielding such as an Acolyte. If using the Sentry Turret, it's best to focus on damage per second abilities, i.e. the "Flamethrower", "Armor-Piercing Ammo", and "Shields & Damage" evolutions. If you find the turret to be a little too fragile, weak, or just useless, consider maxing the other four skills. By combining Cryo Blast with Incinerate (and the "Frozen Damage" evolution at rank 6 on Incinerate), you can inflict huge damage to frozen enemies and armoured enemies. It's also possible to sometimes create cryo and fire explosions.
Thanks to the turret, the quarian excels at holding chokepoints. Find a well-enclosed spot with some cover, deploy the turret and, when possible, let the enemies come to you. However, don't let the turret do all the work; keeping up the gunfire helps mow them down.
It's worth noting the turret will target cloaked enemies. If you're defending an enclosed area, the "Flamethrower" evolution becomes a warning system and ally against these enemies.
Cerberus enemies are mostly shielded, but Cryo Blast will still slow enemies down by 15% or 35%. It also makes them vulnerable to taking double damage from Incinerate with the Rank 6 evolution "Freeze Damage", as with all enemies. Those that are shielded should ideally be tackled with a team mate with Overload, Energy Drain, or a powerful anti-shields/barrier weapon like the Reegar Carbine.
Against Atlases the Cryo Blast-Incinerate combo is extremely deadly since it allows for over 1000 damage to be inflicted with just one Incinerate. The turret is a simple fire, forget, and hope it kills as many as possible strategy. On higher difficulties, this is very difficult.
Collectors are a challenge for Quarian Engineers, since all enemies can gain barriers through possession, but otherwise a Quarian Engineer can take on the Collectors fairly easily. A high rate-of-fire weapon, like the Tempest is very useful. The Acolyte also works here.
Cryo Explosions are ultra effective against crowds of enemies, freezing unprotected enemies, and chilling the rest, potentially heightening Incinerate's damage (if Freeze Combo evolution was chosen), and Fire Explosions are achieved by deploying the Sentry Turret (with Flamethrower evolution) and using Incinerate to detonate. All are great for crowd control.
Use of cover is largely unnecessary since you will be evading multiple aggressors at once most of the time.
Geth are possibly the most difficult of all the forces, since every enemy (with the exception of the Geth Trooper) has some kind of shielding, preventing snap-freezing. Since most geth are shielded, it would be highly recommended to either bring the Acolyte, or abandon the Quarian Engineer and instead go for one of the other types of Engineer.
While Cryo Blast does slow enemies down and can be combined with Incinerate, the amount of times you will be stunned outside of cover by geth makes it very difficult to fire off powers against them.
The only enemies that Incinerate is effective against are Geth Pyros and Geth Primes, but they need their shields stripped first.
Reapers are the ideal enemy for Quarian Engineers. With only two enemies with extra layers of protection (Marauders and Banshees) it is very easy to snap-freeze the unprotected, or chill and slow the others. Because the enemies cannot dodge (except for Marauders) Cryo Blast is very effective. Incinerate is also deadly on its own, but together they pretty much lock enemies down in a more permanent fashion.
Only Banshees are a real nuisance because they are heavily protected by barriers, and a very high amount of armour. Their teleport ability also acts as dodging tactic for your bullets and powers, causing a great deal of stress, but if you pair up with somebody with shield-draining powers, the barriers can be brought down quickly, to expose the Armor to Incinerate.
Brutes are a pain, but are slow, cannot dodge attacks, and are highly vulnerable to Incinerate and powerful-but-slow weapons.