Type of Attacks
Bosses will only use certain attacks at programmed points in the battle. For example, the player will learn of a different type of attack once the boss' life has been reduced to half. Identifying which attack will be used at a certain point makes it easier for the player to plan against an attack. Some attacks also have blind spots -- these are parts of the screen where the attack won't reach, making the player safe from that attack. Learning the existence of these blind spots makes surviving much easier.
Dodging Attacks
Knowing how to dodge a boss' attacks is the best way to stay alive long enough to be victorious. At first sight, a boss may seem quite imposing due to the number of projectiles thrown towards the player's character. The important thing to remember is that most boss attacks follow a pattern. In other words, what may at first look like an unavoidable wall of bullets is simply a labyrinth that must be solved in order to survive the attack. Think of the attacks as nothing but mazes and the first step has already been reached.
Concentration Strategy
It is one thing to recognize there is a way out of each attack; it is a completely different matter to maneuver through hundreds of projectiles without being hit. The best strategy is to completely focus on the controlled character and ignore anything else taking place on the screen. Trying to think ahead and plan an A-to-B route through a boss attack won't work, as there is too much that can't be predicted in this situation. The structure of the attack may change in an instance, making a planned route useless. Concentrating on the character will make the immediate route much clearer and will have all the blind spots covered.
Weak Spots
Every boss has a weak spot that must be attacked to bring it down. As a lot of the Soldner X bosses go through multiple changes, the weak spot will often shift. The way around this is either by looking for obvious weaknesses, such as exposed eyes, or just firing upon every part of the boss and finding the weak spot by trial and error. Be patient with weak spot targeting: It may be frustrating to have to dodge away from attacks that prevent full concentration on the targeting of the weak spot, but it's important to remember the boss is able to take much more damage than the character.