Assault Suit Leynos

See "Attack Ship - Master Hunter". This trophy works the same way, except you only need to destroy the boss ship before the countdown timer reaches 30. This is much easier than 45 and you should be able to do this in either Arcade or Classic mode without too much effort. Just make sure you're right at the end of the stage when the boss appears, so you have the maximum possible amount of time to bring it down.