Destroyer is the easiest mech boss. Terraria master difficulty

This is a proof of concept done in a Journey world on master difficulty with no armor or accessories, base/unreforged star cannon, 50 fallen stars and the slime saddle. Not sure if it’s absolutely necessary, but this was also a large world to give more time while falling. To recreate this, just create a hole to the bottom of the world (if you already have a hellevator, that works just fine) and jump down soon after summoning the destroyer. Wait a while for him to fully spawn in and straighten out, occasionally using the slime saddle to fall faster away from him, and use mini map overlay mode to line up your aim more easily. Then just shoot your star cannons and the pierce will just eat him up in seconds.