This way I can aim with my right thumb, and fire with my left. It's weird, but it works. The advantage is that you can be super accurate, and pull off headshot after headshot. The disadvantage is that you can't move while shooting. That means it's rather useless for the fast-paced world of multiplayer. But it works well with the more deliberate pace of singleplayer. And for turret, breach, and sniper missions where you can't move anyway? It's perfect.
You can also try the third control method (hit 'controls' in the settings and press '3' under the picture of a phone) where you move with the left stick, aim with the right, and shoot by tapping the middle of the screen. It's fiddly, but can work well. And if you're really struggling, turn on 'auto-shoot'. Now, your weapons will automatically fire when you aim at an enemy. It only works in singleplayer, and it will make the game quite easy, but it's good for beginners. And finally, you can turn on 'gyroscope' if you'd like to aim by wiggling your phone, and 'Invert Y-axis' if you reckon up means down and down means up because you are an aeroplane.