The Quarian Marksman is a class capable of dealing a lot of damage in few seconds while rendering enemies defenseless. Combining the increased accuracy and rate of fire of Marksman with the stacking damage bonus of Tactical Scan and Sabotage (if evolved to do so), the Quarian Marksman is especially useful for taking down large enemies like Atlases, Banshees and Praetorians.
Because cooldowns remain important to this class, weight should be an important factor in weapon selection.
Sabotage remains a highly situational skill; it is useful against Cerberus and geth, but less useful against Reaper or Collector forces. Sabotage may not be worth investing all 21 points into because it competes with Marksman and Tactical Scan for cooldown time and because it remains a situational power.
The Tactical Scan, Marksman, and Quarian Defender powers all have evolutions which increase weapon damage-per-second and headshot damage. High-accuracy rifles, shotguns equipped with the Smart Choke mod, and heavy pistols with the Cranial Trauma System mod are recommended for a Quarian Marksman who specializes in headshots.
Tactical Scan can be evolved to provide a maximum of 32.5% extra power damage against enemies, and Sabotage's Rank 6 Tech Vulnerability evolution increases tech damage against a target by 50%. Combined, these two debuffs allow squadmates focused on tech powers to inflict devastating damage against strong targets.
The Quarian Marksman has 100 less shields than the other quarian classes so he is a little bit more flimsy. To compensate, an update was put in to increase the damage they can potentially inflict. When all weapon upgrades to damage are obtained, a 30% damage boost is given to weapons, where other Quarians would only gain a 22.5% damage increase.
Much like the Turian Soldier, your main power should be your unique one - Marksman. The way to equip yourself though is very different. A sniper will not gain much use from Marksman, instead focus on a powerful anti-armor Assault Rifle or shotgun, and your secondary as a power anti-barrier/shield weapon like the Acolyte.
Tactical Scan, combined with Marksman, can make short work of heavy enemies such as Scions and Praetorians.
Sabotage is a bit useful against troopers and captains, because they have firearms.
Because Quarian Marksman depends mainly on weapons over powers, disabled powers make fights only a bit less effective.
Sabotage can hack all geth units, but its long cooldown limits power usage. However, it is useful to delay the actions of dangerous enemies with deadly ranged attacks such as Geth Rocket Troopers, Geth Bombers and Geth Primes.
The Quarian Marksman shines against the Geth. Geth heads are easy to target to ensure maximum headshot damage and Sabotage will not only turn any enemy against their former allies but also provide a safe period for the Marksman to fire upon a hacked enemy without fear of retaliation from their target.
Tactical Scan will kill Geth Turrets and enemy Drones instead of causing its normal effects.
The Tactical Scan evolution Area Scan can also be used to reveal where Geth Hunters are - however, its effects wear off quickly.
Tactical Scan can be used to fantastic effect on Geth Primes - it allows a player to then easily strip its shields with Marksman, and then to kill it with a weapon upgraded with anti-armor ammo.