The Assassin is an upgraded Sniper, that can shoot farther, has a bigger FOV (Field of View) and deals more damage with each shot. On the downside, it will move slower. It features an even longer barrel compared to the sniper.
Keep a safe distance: Don't let anyone get close to you or you will die.
Predict the future: (or lead target) Always fire ahead of them when they are on the move.
One down, Two to go: Concentrate your fire at one target at the time.
Assassinate: You have to kill the enemy target when they are not paying attention or being distracted.
Start the hunt: Farming using Assassin is almost impossible when your fire rate is that slow. Try to hunt the enemy when you reach the Assassin. If you are not prepared, you can wait until level 33 or level 35 then go.
Use your advantage: Due to your larger FoV, you can spot tanks before they spot you. Try to use this to benefit yourself.
Close combat: When using the Assassin, you have to make sure to keep some distance between you and your enemies. When using this tank, you're deadly from afar, not up close. If enemies try and get close, eliminate them as quickly as possible.
Charging in: You will die (or survive with a chance of 0.2%) unless they make a terrible mistake.
Isolate yourself: Unless you are in FFA, isolating yourself means dead. Not to mention that enemy tanks can come in groups and kill you in a split second
Priority target: The Assassin is either the weakest link or the most dangerous enemy in the opposite team. Kill them, you will reduce threat for your team and let you level up easier.
Rush in: But try to wiggle your way in. Assassin can be vulnerable if closing distance but do not under estimate their power.
Unpredictable movement: Increases chance that you will dodge the Assassin's shots more often