The Galleon can be a very powerful ship when it is flown and positioned well. However it also has its weaknesses, these include:
Blind spots: Above, below, front, behind.
Ship Design: Heavy weapons don't have the best arcs. Easily disabled. Long rebuild time. 2/4 have arming times so when you're close they become ineffective. Balloon is very large, along with the hull. Lastly ship components are fairly spread out and can make it difficult to keep operational under fire.
With approaching a galleon, there are two mindsets. a. Do I want to use close range to kill it? Or b. Do I want to out range it?
If a. Observe the map and see what clouds/terrain you can use to get close. Next pick a blind spot, this will force the pilot to move his ship accordingly. Most of the time you'll be able to manipulate the galleon into doing what you want if you try and stick to the blind spots. E.g. If dealing with a galleon sitting at the height ceiling I'll try to remain hidden on the approach. Once close I'll attack from underneath most likely at the rear where the turning engines are. This will force the galleon to turn/move because he wants to kill me. From here I readjust my ship position to stay in the blind spots, this can be done in various ways. The most important part is you must apply pressure. E.g. Shooting out turning engines then pounding away, popping the balloon repeatedly.
If b. There are many ships that can effectively engage a galleon from long range and either kill it or disable it effectively. Having said that this can take some time but it's generally the safest option. Just make sure to maintain a high situational awareness for his team mate, in case you need to change targets.
In your context of a CP match, I assume it was Labyrinth you were playing on. In which case you may be able to use a height advantage to approach. Or perhaps with your ally bait him into changing his gun arcs then focus him down from different angles, forcing him to reposition constantly. (Just make sure these angles aren't broadsides.
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