I have tried to kill the Destroyer a couple of times. The first time I built the arena it killed me and I went back to my spawn point. It followed me and killed all of my MPCs. I don't use guns very much because I have terrible aim. I have a adamintite sword and mythril armor. What different kinds of weapons should I use since these are not working for me?
I highly suggest the Nimbus Rod it's insanely effective against The Destroyer (and other multi-segmented enemies).
You can acquire them by killing Angry Nimbus's which appear on the surface when it's raining.
After acquiring only one rod, you can summon two clouds and then drop the item for your friends so that they can summon two more clouds each.
Use any other preferred weapons to augment the damage from the clouds, but the clouds will be your main source of DPS. You can even get the crimson rod for another cloud (although it does less damage).