The Obi-wan method
You know when Obi-wan said, "I have the high ground." Well, in Ninja.io, it is vital. It allows you to easily surprise attack your opponents who are focused on their fronts. You do not necessarily need the tallest ground available, you just need the best spot on the map. It seems the Obi-wan method could also be described as camping.
if you understand the physics of Ninja.io and analyze your character's movements, you can bop their heads without a care in the world. You should know when your opponent should shoot and where they will run, and act accordingly, eg. if your opponent is running with your flag down a tunnel you should block off the other end so that they will have to turn back towards your other teammates.
Gravity in Ninja.io is nothing compared to real life. You could ask the developers or do it yourself, to estimate the gravity in Ninja.io. (I mean, you can fly?! There's a UFO on the Canyon map that either bops you down at a hundred miles per hour or kills you when you hit it? This is why ninja.io is one of the best games!)
Acceleration in Ninja.io is not needed to estimate, because it may be an advantage, instead of a value. It may help you as far as to win your team a game of Capture the Flag!
Double-shooting can be done with many weapons. Some combinations that are worth trying are:
To get two Barretts / RPGs you have to find the guns in the map. Maybe you will be able to get them from points you died at and consequently dropped the relevant guns / weapons you used then.
Barrett and RPG
It does not matter which order you shoot the weapons in but to shoot the RPG you need to prone or crouch (X and S respectively). You will also need to press Q to switch weapons. It is not necessary to stop proning or crouching when you shoot the Barrett. Optimise your actions. The same applies for M79 and RPG.
You do not need to prone or crouch for this, but you need to click and press Q quite fast.
For this it is necessary to keep proning or crouching when shooting both. You only need to press X once, but you will need to hold S.
Keep in mind only the M79 has a fast enough reload to keep your shots always ready to use. Barretts and RPGs have very slow reloads, perhaps the slowest, making them a risky weapon to hold, but does not alter their effectiveness.
The M79 launch is a great advantage in speed, but a disadvantage in health. Losing health depends on where you shoot. An ideal strategy is to jump with W and immediately shoot in the opposite direction of desired travel and your loss of health will be minimised.