Flying knife is awesome for destroyer, just hang out in a 20*20 (30*30 might be easier to dodge him, but it means the flying dudes have more space to attack you from) box with a 3 block gap or something at the bottom to attack through and spam the flying knife through his body.
Nimbus staff above the gap adds a lot of damage if you can get it/have it since it only needs refreshed every minute and will be pretty much constantly hitting several parts at a time.
If you cba farming mimics for the flying knife, anything that pierces should be fine tbh, that was just the best weapon I had at the time when I did it.
Just make sure you don't get hit by his head, always dodge his head even if you know it means running through his body to get safe afterwards, his head does 3 times as much damage as his body.
Finished all the bosses a couple days a go on expert, was surprised how easy they all were (apart from Fishron, he was more the level I expected) considering how terrible I am (I die all the time to normal mobs and died a ton to normal mode bosses). The only hardmode boss I legit died to was Moon Lord (once) and Fishron (quite a few

), I also died to Destroyer and then thought the fight was super broken and insanely difficult, then I realised I was wearing my fishing set

, and once to Plantera because I had huge fps lag the first time I spawned her and basically lost 60% of my hp in the first 5 seconds because I couldn't move properly.
The fire pillar should have spawned after killing Moon Lord IMO, much harder final boss, so many deaths until I started just stepping in and out to spawn a few dudes at a time

, but that's super boring, need to come up with a better way to do that I think.
Still need to do Frost Moon, did Pumpkin Moon earlier and got about 13k points iirc.