To kill a boss, you are always looking for high single point damage, even if it is a boss with multiple guns, turrets etc. This is because the enemies’ guns, always continue to function, even if they are damaged. You must take out the entire gun, to stop it from firing. Because of this, it is beneficial to take out one gun at the time, rather than damaging several.
Brobots with excellent boss killing abilities as there special weapon are e.g. Nayad, Scarab (though a bit difficult to handle), Beetle, Hornet (slow reload though), and Nicolas XXS. If you are not scared to get up close Cetus and Kraken are very good.
Always consider bringing a speed enhancing brobot to any battle, and most speedbots actually do decent damage to aces. This holds for Legatus, Menace, Risk, Hazard, though Doom seems to do little to aces, despite a single impact beam.
Then you can have a different strategy: Focus on using the brobots to enhance power, armor, etc. There are also brobots increasing damage dealt to aces, like Hazard.
Just as an example Hazard + Nayad is a very good combo. You will have two decent bosskillers, one speed enhancing, and one power enhancing. Both brobots regenerates armor (if the boss fight is long), and Nayad reduces incoming damage. All in all – you will be very well equipped against all bosses.
Another example is Menace (or later in the game Legatus) together with Nicolas XXS. You will again have one speedbot, but this time paired with an armor bot. Both have good weapons on their own, and good perks as well.
In all – there is no easy answer, or rather, there are many good answers.