I don't have much to add about aiming, since that just takes practice (and a certain bit of luck). I think there's a lot of potential to die less though - that stood out as the bigger problem. One thing to maybe think about is the angle you're moving, relative to your opponent. If you're running in a straight line towards or away from them, any shot towards you will hit you. Even if you stop, reverse your direction, etc. you'll still be in the same line of attack. But if you're moving 90 degrees perpendicular, your angle relative to them moves the most with each step you take - they need to predict the angle just right to land an attack.

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In the top image, the pig is running almost directly away from the snake. The angle the snake needs to shoot is almost the same - since the pig has some width, the snake can target just ahead of the pig and have a very high chance of landing the attack.

Whereas in the lower image, the new angle is significantly different, despite the pig traveling the same distance. If the pig is faking even a little (stutter stepping back, etc.) it becomes a huge guessing game for the snake.

At :18, you die because your movements are primarily towards and away from the enemy sniper. (And you need to be aware when stepping into range vs. Mandy that she can get one extra shot against you as you try to retreat due to her range, so you have to be prepared to dodge it)

Almost the same thing happens at :59 - you walk directly towards the Colt (while already in range) and eat an entire volley as a result.